Can I undo the creation of this merchant republic?

So I’m a King and I cajole a foreign courtier – call him A – who has a claim on an independent duchy D to come join my ever-so-much-more-prestigious court instead. Following roughly the guide in this question, I want to press this claim, so I look at what land I can give the claimant. A city C happens to finish construction in county B (which is about 6 counties away from the target duchy – totally unrelated, even a part of a different de jure kingdom) right around this time, which gives me a ‘Wrong Holding Type’ and ‘Demesne too Big’ warning. So I figured let’s kill all these birds with one stone. I make the claimant duke landed by giving him the city, declare war with his casus belli and am off to the sieges.

A short time later I finish squashing the independent duchy into the dirt and win the war. Much to my surprise, a new merchant republic is formed in the formerly independent duchy. The Lord Mayor A is still mayor of C and remains my vassal, but the Duchess of D is still around as a countess of her duchy’s capital. Lord Mayor A doesn’t control any counties, just a city and a duchy.

First real question: how did this happen when most merchant republic vassalization guides say you need to have the same character hold a city and a county in their de jure duchy before giving away the duchy title?

The downside is I already have a vassal merchant republic! What’s more, they already have trade posts up and down the coast, including in the county seat of the new duchy. I gather this is a bad situation, as trade empires compete with each other? And get a stronger bonus from controlling a longer swath of sea lanes?

Second real question: is it so bad to have multiple merchant republic vassals? I should do everything I can to keep them from competing with each other, including total dissolution of the new one, right?

Third question: How can I best accomplish this? Can I revoke the city title and turn Lord Mayor A into a regular vassal (somewhat like this question says)? Would that dissolve the new Republic? Would I need to revoke the former Duchess of D’s barony and give it to the Lord Mayor?

Edit: I don't think any of the answer's given so far will work. Here's the options the game won't let me do.

1. Revoke the County of D. The former Duchess is now a Countess and it says I'm her liege, but the option doesn't appear when I right click her portrait. I do have a truce with her for the next 10 years: the title screen for the county implies I might be able to revoke it when the truce expires.
2. Revoke the City of B, which is where A is the Lord Mayor. Oh, well technically it says he's the Grand Mayor of D now. Right click him -> Revoke: no titles. City of B title screen: revoke button says "Not Revokable".
3. Grant A any other county. My entire kingdom is only 19 counties, so now that I have 2 counties under republic control, I can't grant a 3rd as I'm over the 10% limit.
4. Usurp or revoke the new Republic of D title. It's no longer a Duchy title; apparently this is not allowed for the primary title of a republic.

On a cheerful note, the Republic of D doesn't yet have any trade posts. I'm pretty sure there was a post belonging to my older republic in county D before the war ended (it was sieged a few times - not sure if that destroyed it or the creation of a new republic in its backyard did). Another positive is that having pressed A claim on D gives him a crazy good opinion boost towards my King, so I could probably slap him around without making him hate me, if I could find an option the game will allow. Of course, that opinion boost will probably also expire as soon as my King kicks the Royal chamber pot.

So for now it seems like my options are to live dueling republics, wait 10 years, quickly acquire more counties to grant an additional one to republic control (which I'm now thinking won't actually elevate Grand Mayor A to a feudal vassal status) or back the game up to an earlier save and find a different holding to grant to A to press his claim on D.

New question: do truces expire on succession? Because that will probably happen a lot sooner than 10 years...

Clever, you found a way to create a merchant republic I hadn't considered before.

First, I think the only reason you need the soon-to-be Grand Mayor a county before granting a duchy is because the game won't allow you to grant a Baron-tier character a Duchy directly. Apparently you can get around that with claim wars. The city doesn't actually need to be in the county, nor the city or county in the duchy, it just makes for cleaner maps and less arguments between vassals over who should control what.

Second, unless crown laws (Medium or higher) prohibit it, the Grand Mayors are likely to try declaring trade wars on one another and possibly get Feudal Dukes (or even outside powers) to Embargo the other republic. They will also likely be plotting to steal each others trade posts and if the new republic gains enough posts they could threaten the old republics trade zone bonuses. That kind of internal strife costs money (and thus is inefficient), so it's probably better to quietly smother your fledgling republic.

Third, I don't think the game will let you revoke the city if he holds no other county or baron tier titles. You might be able to revoke the city if you grant him a feudal county then strip him of the city title. Of course, then you've created a Duke with a reason to hate you, and the two unjustified revocations will give you a -40 relation penalty with all your vassals. The simpler option, which will offend your vassals less (-20 instead of -40), would be to simply revoke the Duchy directly. The former leader of the republic will go back to being a regular Mayor and you will have a Duchy to hand out as you see fit.

Yes, you can remove the merchant republic. Revoke the lord mayor's duchy. If he controls no duchy or higher level titles, his republic will be destroyed. You can then give the duchy to a feudal character- most likely one of the other counts in that duchy.

Whether you need to do this depends on the situation. In the long run, one enormous republic will be stronger than two competing ones. However, in the short term, you will lose a lot of income. All the trade posts from the destroyed republic will be destroyed with it and it will take a lot of time and money for the second republic to replace them. It also depends if the second republic is even in a position to do that- if there's another republic, outside your control, they will also take territory from this.