When an extension registers itself as belonging to multiple categories, its row is inserted just as many times with the same HTML id, causing validation errors:
http://validator.w3.org/check?uri=http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FSpecial%3AVersion

Possible solutions include but might not be limited to:

• do not output IDs for extensions at all
• only output IDs for the first occurrence of an extension
• append/prepend each ID (or duplicate ones only) with the name of the respective category