I know I've seen in several places on the forums that you are not allowed to use two actions with the same subtype in the same turn (notably, two Standard Attacks in the same turn). But for the life of me I cannot find this anywhere in the rulebook or the errata. Could somebody please point me to where this is written?

Generally the stipulation on subtype is: You cannot use more than one action with the Attack subtype each turn. (Using normal actions, talents - Furious Charge - and special rules - Squad/Solo modes - for instance, bend this rule.)

DW:Core Page 236

"A character cannot take the same Half Action twice in the same Turn."

DWL Living Errata 1-1 Page 4

"... the power gains the Attack Subtype (see page 237), and thus cannot be combined with other Attack Subtype actions (such as Full-Auto Burst or Semi-Auto Burst, and so on)."

I think it was only Dark Heresy that specifically disallowed multiple actions with the same Subtype, but I may be mistaken. The only Subtype that is specifically disallowed from taking multiple actions in later rulesets is the Attack Subtype.

It's not very consistent, because as Herichimo notes, there are Talents and such that may very well bend this rule considerably.

Note that while on the topic of Actions, it should also be noted that although never listed as a Subtype, there are additional rules preventing the use of more than one Psychic Power per Turn (or Round..?).

May I ask in what context you are asking? I can only assume that it's a mild argument in the group as to what combinations of actions can be used.

I do believe their conundrum revolves around making two Standard Attacks during a normal Turn. Which is disallowed as a character can not use any specific Half Action more than once a Turn (p. 236 DW:C) nor make more than one Attack Subtype action a Turn.

The Dark Heresy book and errata, as far as I can tell, make no mention of being unable to use two of the same subtype action a Turn. BUT it does state a character may perform only, "one Full Action or two different Half Actions." [Emphasis mine, p. 188 DH:C] Which is the same restriction in place for Deathwatch.

Since the only attack action which is a Half Action is the Standard Attack (minus talents) the inability of using the same Half Action twice keeps the combat as the designers intended. When the talent Perternatural Speed (which did not exist in Dark Heresy or Rogue Trader for that matter) is included the potential for abusing Multiple Attacks Action AND a Standard Attack (against the intent of the designers) arose. So they introduced the limitation on Attack Subtypes to keep things at a sane level.

The Rogue Trader Errata (ver. 1-4 at the least) has the intended errata which is still missing in the Deathwatch errata (though obviously intended as shown by text in Psychic Powers entry listed in my first post). I will provide it below, but we can chock this up to an oversight, combined with the lack of an updated errata for over a year.

Entry from Rogue Trader errata:
Using Actions (page 237): Under the heading “Using Actions” add the sentence “An Explorer cannot perform more than one of the following during his Turn: an Action with the “Attack” subtype, using (not sustaining) a Psychic Technique, or using a Navigator Power.
“The only exception to this is that a character may use a Psychic Technique or Navigator Power that only requires a Free Action during his turn and still perform an action with the “Attack” subtype on the same Turn. They may not use an additional Psychic Technique or Navigator Power during that Turn.”

It is interesting that both Rogue Trader and Deathwatch have their Combat Actions tables on the same page.

Yep, it's a common understanding, inferred from other systems and references in the errata which seem to assume the rule is present, but it's a rule that's just plain missing in deathwatch. Note that the rule only makes sense, so even if you don't accept that it is present in the rules as they stand, GM has ultimate fiat, so all they need to do to correct this is just say the rule exists and it does. Anyone not being deliberately obstinate will be operating under the understanding this rule is in effect, so for convention's sake, when discussing anything with others, I recommend also assuming this rule exists.

Sorry for not responding (didn't get email notifications for some reason): thanks y'all! It was just general confusion in the group that arose when one guy tried to do a single fire twice in one round, and most of us remembered that you can't do that, but we couldn't find the specific reference (it's not in an easy to find place/where you expect it). Thanks again guys!