We’d get this error if we are trying to import a solution whose upgrade solution is already there in the target system.
So, either we need to Clone the Solution to remove the upgraded solution or we need to delete the Upgrade Solution. Deleting the Upgrade solution will only delete the solution it will not delete the components as it will already be there in the main solution whose upgrade has been imported.
Hope it helps..
I also encounter the same error when I manually updated the solution version of the patch solution and the first two digits of the patch solution (X in this sample X.X.0.0) does not match with the first two digits of the base solution in the target environment.
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