I'm guessing no, but can you trigger the crit effect of a weapon more than once on a given attack?

Yes. One by spending the boons and then also with the Sigmar comet.

No.

You can score multiple critical hits on an enemy thanks to Sigmar comets, success/boon lines on your action card and the crit effect of your weapon but you cannot activate each one more than once.

For example if you were attacking with "Insulting Strike" and rolled 1 success, 4 boons and a Sigmar's Comet you could activate the boon line for a crit, activate the weapons crit rating and score a crit via Sigmar's Comet (3 crits!) but you couldn't (for instance) score 3 crits by activating the weapon crit twice and the Sigmar's Comet once.