This paragraph from the metadata format section ...
The keyid MUST be unique in the "signatures" array: multiple
signatures with the same keyid are not allowed.
... seems to conflict with these paragraphs from the metadata format section ...
Note: The "signatures" list SHOULD only contain one SIGNATURE per
KEYID. This helps prevent multiple signatures by the same key
... and the client workflow section ...
Even if a KEYID is listed more than once in the
"signatures" list a client MUST NOT count more than one verified
SIGNATURE from that KEYID towards the THRESHOLD.