fix bind:group with duplicated values

[tanhauhau]

Fixes #4397

• using set to prevent duplicate
• passing in the changed input .__value and checked to determine whether to add or remove the current value, to not to be affected by its duplicate.

side note: this seemed adding more code to fix an edge case, i was wondering should we have flags for certain bugfixes, so user can opt-in more code for fixing things, while majority can stay slim.

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