We got this issue while importing (managed) solution from dev to test organization. We had recently deleted one of our existing 1 – n relationship and had created a new n – n relationship between those entities. The issue was that the new relationship had exactly the same schema name as the old relationship which we deleted.
The solution was to delete the newly created n – n relationship in Dev and create a new one with different schema name. After this change we were able to import the solution successfully.
Hope it helps..