Does a divorce take effect if I intend with a firm intention to divorce my wife upon doing a certain thing, and then I do this thing under duress, and does the divorce become repeated with the repetition of this action?

If the matter is limited to the intention of divorce without uttering it, then divorce does not occur. Divorce occurs through utterance or writing with intention, or by the gesture of a mute person.

If you utter a divorce and condition it on doing a specific thing with the intention of divorce, then it occurs when that thing is done, unless the action was due to forgetfulness or coercion, in which case the divorce does not occur. If the divorce does not occur due to forgetfulness or coercion, the condition remains valid, and the divorce occurs if you do the action later.

If the wording was, "If I do such and such, my wife is divorced," then a single divorce occurs. However, if it was, "Every time I do such and such, my wife is divorced," then the divorce is repeated with the repetition of the action.