If you control two Arcane Signet does the second one still tap for mana in Commander?

The legend rule (CR 704.5j) only applies to legendary permanents that share the same name — but Arcane Signet is not legendary. It is simply an artifact with no supertype of 'Legendary,' so the legend rule never triggers and both copies remain on the battlefield simultaneously.

Each Arcane Signet has its own tap ability that reads: '{T}: Add one mana of any color in your commander's color identity.' Each copy can be tapped separately, giving you up to two mana per turn cycle from the pair (CR 605.1 governs mana abilities).

There is also no Commander-specific rule that restricts you to one copy of a non-basic-land, non-legendary permanent. The 'one copy per deck' restriction in Commander (CR 903.5a) is a deck-building rule; once cards are legally in your deck, having two copies enter the battlefield raises no rules issue.

Concrete example: You control two Arcane Signets in a Ur-Dragon (WUBRG identity) deck. On your turn you tap the first for {G} to cast a mana dork, then tap the second for {R} to cast a hasty creature — two separate mana sources, both fully functional.

Bottom line: both Arcane Signets tap for mana without interference. The legend rule does not apply to non-legendary permanents (CR 704.5j), and Commander adds no further restriction here.