Currently, we have the following three tags on Stack Overflow that all refer to the same concept:
Two of them should be made synonyms. I don't know which tag would be best as the master tag, probably idempotence, as that is the name of the concept.
Only the idempotent tag has a tag wiki, so that needs to be copied into the new master tag.
EDIT: The above was not my suggestion. The powers-that-be have put things into my mouth here. I agree with the suggestion to retag the 8 other questions to [idempotent].