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The existing block has not been overwritten – thanks to MultiBlocks both exist in parallel: https://www.wikidata.org/w/index.php?title=Special:BlockList&wpTarget=%7E2026-24976-02
Per T419949#11921313 this appears to be intentional – and I don't think it's much of an issue?

OK, I think that doesn't make sense.
On Special Central Auth, for example, only the current block is displayed and the block hiszory in the log is also unnecessary long.

It sounds like you mean the TA massblock SHOULD overwrite existing blocks? If so, we will probably need to require user interaction when the target TAs already have multiple blocks, since there would otherwise be no way for the system to decide which block to overwrite. This could interrupt the administrative workflow to some extent, so this likely requires a deeper discussion.