Rename the offending blocks. The error is generated when you insert a block called xyz into a drawing called xyz. Initially there is no error generated, however, if you try to subsequently insert the xyz drawing into another drawing, the error is generated because there will be two references to xyz in the third drawing. One for the inserted block, another for the nested block, if this were allowed to take place, you would have a situation similar to placing two mirrors perpendicular to one another, the reflection would go on to infinity, as would the block definitions.