DeclarationError: Identifier already declared. Two imports import the same ERC721 contract

Question

Im building a contract that uses two contracts that both use the ERC721 zeppelin contract, when I import the two as such:

import nonFungableWidthAddedFunctionality1.sol;
import nonFungableWidthAddedFunctionality2.sol;

I get the following error:

$PATH DeclarationError: Identifier already declared.
import "./../nonFungableWidthAddedFunctionality2.sol";
^--------------------------^
$PATH The previous declaration is here:
contract ERC721 {
^

Both of these contracts need the ERC721 implementation since they are also used separately.

Answer 1

You are declaring contract ERC721 more than once, thus the error.

Essentially, you're doing this:

contract Contract1 {
}

contract Contract1 {
}

Answer 2

In my case it was caused by collisions between IERC20.sol from an openzeppelin module and my own IERC20.sol file that I had in my interfaces folder. For my own smart contracts I was using my own IERC20.sol, while some of my imported contracts were using the IERC20.sol file found in the openzeppelin module.

As a simple solution I removed my personal IERC20.sol file and instead only used the IERC20.sol interface provided by the openzeppelin module.