Thanks for @loic-sharma 's question in #10628 (comment)

In NuGet.exe command, the package path could be wildcard, so there might be multiple paths after parsing.
But there is a checking on "Name" to reject any existing "Name" at https://github.com/NuGet/NuGet.Client/blob/ac97d923e633f4ee65eec5ec690c4d99d7a922f3/src/NuGet.Core/NuGet.Commands/TrustedSignersCommand/TrustedSignerActionsProvider.cs#L243
So when there are multiple packages, the first one will be added successfully, but the rest will fail for A trusted signer 'Test' already exists.
Apparently this is a bug. We should make the two consistent.
That is, do not accept multiple paths, or, accept multiple 'Name'.