Prevent uploading the same chunk twice

[PVince81]

In case the client sends the same chunk twice from the same file transfer id, return 409 Conflict.

This was observed here #17956 (comment)
It is unlikely to happen in a real environment, but it's always good to have some robustness in place.

The idea is that before rename the "part chunk" to the final chunk, make sure that there is no existing chunk already.

@DeepDiver1975 @icewind1991

[PVince81]

also need to check if how new chunking endpoint behaves if that were to happen.

[PVince81]

Obsoleted by new chunking endpoint. If the client sends the same chunk it would/should have the same name so it would be overwritten.

[lock]

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