One of the complaints I see every now and again is that “Compelled Duel” is really anything but! It’s got a really evocative name but the effect falls kind of flat. This spell attempts to rectify that by adding a variable cast time, similar to how Plant Growth works. While the practice of dueling is not necessarily legally binding, flouting an established challenge may affect your social standing in high society and cause dramatic shifts in a campaign’s story!  

The historical Code Duello was a set of rules developed in 18th century Ireland in an attempt to establish “fair” practices for settling grievances of honour. It was adopted shortly thereafter by most of the English-speaking world, though dueling remained a subject of questionable legality in many counties. Here are the first 10 rules of the Code Duello (sourced from PBS.org):

 Rule 1. The first offense requires the first apology, though the retort may have been more offensive than the insult. Example: A tells B he is impertinent, etc. B retorts that he lies; yet A must make the first apology because he gave the first offense, and then (after one fire) B may explain away the retort by a subsequent apology. 

Rule 2. But if the parties would rather fight on, then after two shots each (but in no case before), B may explain first, and A apologize afterward.

N.B. The above rules apply to all cases of offenses in retort not of stronger class than the example.

Rule 3. If a doubt exist who gave the first offense, the decision rests with the seconds; if they won't decide, or can't agree, the matter must proceed to two shots, or to a hit, if the challenger require it.

Rule 4. When the lie direct is the first offense, the aggressor must either beg pardon in express terms; exchange two shots previous to apology; or three shots followed up by explanation; or fire on till a severe hit be received by one party or the other.

Rule 5. As a blow is strictly prohibited under any circumstances among gentlemen, no verbal apology can be received for such an insult. The alternatives, therefore -- the offender handing a cane to the injured party, to be used on his own back, at the same time begging pardon; firing on until one or both are disabled; or exchanging three shots, and then asking pardon without proffer of the cane. 

If swords are used, the parties engage until one is well blooded, disabled, or disarmed; or until, after receiving a wound, and blood being drawn, the aggressor begs pardon.

N.B. A disarm is considered the same as a disable. The disarmer may (strictly) break his adversary's sword; but if it be the challenger who is disarmed, it is considered as ungenerous to do so.

In the case the challenged be disarmed and refuses to ask pardon or atone, he must not be killed, as formerly; but the challenger may lay his own sword on the aggressor's shoulder, then break the aggressor's sword and say, "I spare your life!" The challenged can never revive the quarrel -- the challenger may.

Rule 6. If A gives B the lie, and B retorts by a blow (being the two greatest offenses), no reconciliation can take place till after two discharges each, or a severe hit; after which B may beg A's pardon humbly for the blow and then A may explain simply for the lie; because a blow is never allowable, and the offense of the lie, therefore, merges in it. (See preceding rules.)

N.B. Challenges for undivulged causes may be reconciled on the ground, after one shot. An explanation or the slightest hit should be sufficient in such cases, because no personal offense transpired.

Rule 7. But no apology can be received, in any case, after the parties have actually taken ground, without exchange of fires.

Rule 8. In the above case, no challenger is obliged to divulge his cause of challenge (if private) unless required by the challenged so to do before their meeting.

Rule 9. All imputations of cheating at play, races, etc., to be considered equivalent to a blow; but may be reconciled after one shot, on admitting their falsehood and begging pardon publicly.

Rule 10. Any insult to a lady under a gentleman's care or protection to be considered as, by one degree, a greater offense than if given to the gentleman personally, and to be regulated accordingly.