Initializable - DeclarationError: Identifier already declared.

[dumabg]

Using the wizard, create a contract ERC20 with UUPS.

The resulting code is:

// SPDX-License-Identifier: MIT
// Compatible with OpenZeppelin Contracts ^5.5.0
pragma solidity ^0.8.27;

import {ERC20Upgradeable} from "@openzeppelin/contracts-upgradeable/token/ERC20/ERC20Upgradeable.sol";
import {Initializable} from "@openzeppelin/contracts/proxy/utils/Initializable.sol";
import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
import {UUPSUpgradeable} from "@openzeppelin/contracts/proxy/utils/UUPSUpgradeable.sol";

contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
    /// @custom:oz-upgrades-unsafe-allow constructor
    constructor() {
        _disableInitializers();
    }

    function initialize(address initialOwner) public initializer {
        __ERC20_init("MyToken", "MTK");
        __Ownable_init(initialOwner);
    }

    function _authorizeUpgrade(address newImplementation)
        internal
        override
        onlyOwner
    {}
}

Using Remix IDE, compile the code.

The result is these errors:

TypeError: Derived contract must override function "_checkInitializing". Two or more base classes define function with same name and parameter types.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:178:5:
    |
178 |     function _checkInitializing() internal view virtual {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:178:5:
    |
178 |     function _checkInitializing() internal view virtual {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override function "_disableInitializers". Two or more base classes define function with same name and parameter types.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:192:5:
    |
192 |     function _disableInitializers() internal virtual {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:192:5:
    |
192 |     function _disableInitializers() internal virtual {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override function "_getInitializableStorage". Two or more base classes define function with same name and parameter types.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:232:5:
    |
232 |     function _getInitializableStorage() private pure returns (InitializableStorage storage $) {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:232:5:
    |
232 |     function _getInitializableStorage() private pure returns (InitializableStorage storage $) {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override function "_getInitializedVersion". Two or more base classes define function with same name and parameter types.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:208:5:
    |
208 |     function _getInitializedVersion() internal view returns (uint64) {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:208:5:
    |
208 |     function _getInitializedVersion() internal view returns (uint64) {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override function "_initializableStorageSlot". Two or more base classes define function with same name and parameter types.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:224:5:
    |
224 |     function _initializableStorageSlot() internal pure virtual returns (bytes32) {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:224:5:
    |
224 |     function _initializableStorageSlot() internal pure virtual returns (bytes32) {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override function "_isInitializing". Two or more base classes define function with same name and parameter types.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:215:5:
    |
215 |     function _isInitializing() internal view returns (bool) {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:215:5:
    |
215 |     function _isInitializing() internal view returns (bool) {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override modifier "initializer". Two or more base classes define modifier with same name.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:104:5:
    |
104 |     modifier initializer() {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:104:5:
    |
104 |     modifier initializer() {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override modifier "onlyInitializing". Two or more base classes define modifier with same name.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:170:5:
    |
170 |     modifier onlyInitializing() {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:170:5:
    |
170 |     modifier onlyInitializing() {
    |     ^ (Relevant source part starts here and spans across multiple lines).

TypeError: Derived contract must override modifier "reinitializer". Two or more base classes define modifier with same name.
  --> src/test.sol:10:1:
   |
10 | contract MyToken is Initializable, ERC20Upgradeable, OwnableUpgradeable, UUPSUpgradeable {
   | ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol:152:5:
    |
152 |     modifier reinitializer(uint64 version) {
    |     ^ (Relevant source part starts here and spans across multiple lines).
Note: Definition in "Initializable": 
   --> @openzeppelin/contracts/proxy/utils/Initializable.sol:152:5:
    |
152 |     modifier reinitializer(uint64 version) {
    |     ^ (Relevant source part starts here and spans across multiple lines).

[0xAllan123]

Only import one version of Initializable.
If you’re building upgradeable contracts, always use the @openzeppelin/contracts-upgradeable package, not both.

`
// ✅ Only use the upgradeable version
import "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";

contract MyContract is Initializable {
function initialize() public initializer {
// setup logic
}
}
`

🚫 Avoid Mixing

Do not mix @openzeppelin/contracts and @openzeppelin/contracts-upgradeable in the same contract hierarchy.

Use @openzeppelin/contracts-upgradeable for upgradeable proxies (UUPS, Transparent Proxy, etc.)

Use @openzeppelin/contracts for regular, non-upgradeable contracts

⚙️ If You’re Using Hardhat or Foundry

Run a clean build after fixing imports:
npx hardhat clean && npx hardhat compile
or for Foundry:
forge clean && forge build

[dumabg]

I've changed all the imports to @openzeppelin/contracts-upgradeable and works.

Only a curiosity, why exist import {UUPSUpgradeable} from "@openzeppelin/contracts/proxy/utils/UUPSUpgradeable.sol"; if upgradeable proxies must be use always @openzeppelin/contracts-upgradeable ?

[ernestognw]

Hey everyone! Thanks for reporting.

Recently in v5.5.0 we started reexporting the Initializable and UUPSUpgradeable contracts from the non-upgradeable package in @openzeppelin/contracts-upgradeable:

 // SPDX-License-Identifier: MIT

pragma solidity ^0.8.20;

import {Initializable} from "@openzeppelin/contracts/proxy/utils/Initializable.sol";

and

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.22;

import {UUPSUpgradeable} from "@openzeppelin/contracts/proxy/utils/UUPSUpgradeable.sol";

The rationale was to avoid declaring the same contract twice and to prevent these kinds of issues. However, I just tested the Remix output, and I could reproduce this problem. The reason why this is happening is that contracts imported with a version suffix (e.g. @5.5.0) will import contracts that don't have the suffix, and since 5.5.0 is right now tagged as dev on npm, the contracts importing Initializable upstream will bring back the duplicated Initializable and UUPSUpgradeable contracts.

The easiest way to solve this is to only import from @openzeppelin/contracts-upgradeable as @0xAllan123 suggests, but I would also recommend just adding a remapping to your remix config file. You can do this by using a configuration file:

And then adding the following remapping under settings:

"remappings": [
  "@openzeppelin/contracts/=@openzeppelin/contracts@5.5.0/"
],

Hope this helps

[Amxx]

Note: remix as a tendency to cache contracts. All files previously imported from a (potentially) old version will be used next to next contracts ... and that can create issues. Sometimes the fix is as simple as deleting the cache and letting remix re-fetch the latest version