DeclarationError: Identifier already declared when using Initializable

[0xAllan123]

When importing both @openzeppelin/contracts and @openzeppelin/contracts-upgradeable, the Solidity compiler throws:

DeclarationError: Identifier already declared.
This happens because both versions define a contract named Initializable.

Steps to Reproduce:

1. Install both packages:
   npm install @openzeppelin/contracts @openzeppelin/contracts-upgradeable
2. Import both in a single contract:
   import "@openzeppelin/contracts/proxy/utils/Initializable.sol"; import "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";
3. Compile using Hardhat or Foundry.
   Expected Behavior:
   Either a clearer compiler error or a unified import strategy in docs to avoid confusion.

💻 Environment

• Solidity: 0.8.26
• Hardhat: 2.22.x
• OpenZeppelin Contracts: 5.x
• OpenZeppelin Contracts Upgradeable: 5.x

[kaym0]

@0xAllan123

As far as I can tell, this is a non-issue.

contracts-upgradeable version of Initializable.sol imports the base version from /contracts. So this essentially imports the exact same contract two times.

When importing both @openzeppelin/contracts and @openzeppelin/contracts-upgradeable, the Solidity compiler throws:

DeclarationError: Identifier already declared. This happens because both versions define a contract named Initializable.

Unsure if the root issue is that you're trying to import the entire library for both contracts and contracts-upgradeable, difficult to discern from the above statement. If so, you should not import the entire libraries and instead only the necessary contracts. This is highly gas inefficient amongst other things.

[ernestognw]

Hey @0xAllan123, thanks for reporting. Would you mind sharing a reproducible example? I would like to understand if this is a problem introduced in v5.5.0 or before. In v5.5.0 we reexported the "vanilla" Initializable in the upgradeable package since both are the same contract and having duplicated does in fact cause an "identifier already declared" error.

Our recommendation from 5.5.0 and future versions is to use the Initializable from @openzeppelin/contracts and we'll remove the reexport in @openzeppelin/contracts-upgradeable in the next major.

[ernestognw]

See also this reply in a different thread:
#6078 (comment)

I'm closing this one in favor of #6078