This once was common, but using stamps in actions became so rare that we dropped support for it in Tinderbox Six. There's not much harm in it, though, so we'll turn on support for it once more in 7.0.2 -- the action
invokes the corresponding stamp.
Another approach to avoiding duplication would be to let the agent do all the work. You might have an agent now with a complicated query and action. Split it in two, adding a new boolean attribute $NeedsToBeProcessed:
Query: (something complicated)
Then a second agent performs the complex action:
Action: (some complex action);$NeedsToBeProcessed=false;
Now, your stamp doesn't need to duplicate the action!
In passing, though, I'd suggest that actions that are complicated enough to make one worry about duplication may be Tinderbox’s way of suggesting that you're doing something the hard way!