Wizkids Rules!

Rogue - Unity Squad has the following Teamwatch ability

Teamwatch - Rogue gets +2A.

If I have Rogue active and field another Rogue die, how is the buff applied

A - I choose a Rogue die to get +2A

B - Rogue gets +2A (all active copies, including the fielded rogue)

If B, then how would fielding extra Rogue dice in a single turn work?

Neither option is correct. For Rogue’s Teamwatch ability to initiate, a different Character die with the X-Men affiliation must be fielded.

Reference, from Appendix 1 – Keyword Index of the Comprehensive Rules:

Teamwatch: When a character with Teamwatch is active and you field a different Character die with the same affiliation, use their Teamwatch ability.
(1) Teamwatch counts the number of different active characters, not the number of active dice from those characters. The Teamwatch ability occurs when a different Character die is fielded and doesn’t change if additional identical Character dice are fielded after the ability is initiated.

{Note, this is the updated official definition, which supersedes the keyword definition in previous rule books and the Keyword webpage.}

Additional Discussion
This ruling on Rogue may appear to contradict a previous ruling (see link below) with Starhawk: Precognitive.

viewtopic.php?f=10&t=11332

Starhawk’s ability would enable another Starhawk Character die to receive a +2A modifier because of the specific text, which precludes the fielding of a different Character die condition on the Teamwatch keyword. Had Rogue’s ability stated “that Character die and Rogue both get +2A”, then the fielding of another Rogue die would grant a +2A modifier to the fielded Rogue die as well as to the active Rogue die.

Is that correct? Doesn't a Rogue character die in the reserve count as a different character die with the X-Men affiliation?

Yes this is correct. For the second question, in the context of applying the Teamwork keyword, no.

For reference:

[Old] Teamwatch: When a character with Teamwatch is active and you field another character die with the same affiliation, use their Teamwatch ability.
[Revised] Teamwatch: When a character with Teamwatch is active and you field a different Character die with the same affiliation, use their Teamwatch ability.

Under the old definition of Teamwatch, there was confusion on what the word “another” meant in this context, since it can have two definitions. The first can mean ‘more of the same’, e.g. he had another slice of pizza. The second can mean something different, e.g. she came to the party with another man.

To clear-up this confusion and clarify the intent of this ability, the revised definition of Teamwatch changed “another” to “different”, which encompasses the second definition above. The fielding of an identical character die will not initiate a Teamwatch ability. Therefore, Teamwatch can also be explicitly stated as:

“When a character with Teamwatch is active and you field a Character die that is different from the Character with Teamwatch with the same affiliation, use their Teamwatch ability.”