Agree that both my kids who are diagnosed ADHD do/did this. (One is 16 and has grown out of it!) Of course, I'm not suggesting it is a behaviour unique to ADHD and even if it was, you can't diagnose from a single anecdote/symptom. But in case you were seeing signs in other places and wondering, I'd say it's a potential tick in a box.
I will add that ADHD is very genetic and I have it too, and I think with the "in a minute" example, I can self-reflect that (esp pre medication) I was very guilty of saying this, and in fact what I meant was that I'm engrossed in something (MN probably...), and I don't want to break out of that to go get you a drink or a snack or whatever. So I'd say in a minute, which means go away and stop bothering me, and then if they do that the problem is that I would generally completely forget that the request had ever happened in the first place. Therefore, inadvertantly training my children that asking once and waiting patiently was not a very effective strategy, and what was actually more effective was for them to annoy me to a point where I couldn't focus on what I was doing anyway, at which point I'd stand up and bitch/rant about it (which is in fact attention, which is reinforcing) but I would in fact get them the drink or snack because I recognised oops, I've been avoiding that longer than I intended to. So it was double reinforcing.
What did they learn from those (repeated) interactions?
Asking once and waiting patiently is ineffective.
Asking multiple times and annoying Mummy is very effective.
It's totally valid behaviour (Mum does it) to bitch and rant at someone when annoyed.
I mean, Jesus. If there was a damning self reflection. Of course that's not the parent I INTEND to be, or the messages I want to teach them, AT ALL.
I don't say that in a way to assume that's what's going on in OP's house - just to illustrate that sometimes our unconscious behaviour is more influential than the choices we make. Because in the above example if I had decided that repeated asking would result in some consequence, it would probably not be very effective to squash the behaviour.
So first reflection - as somebody else said, rather than say "in a minute" I either pause what I'm doing to get it straight away, or I give a clearer timescale of when I will get it and then do something to remind me so I don't forget, and then I'm much less likely to get into the state where I'm annoyed by them asking but I realise I have been a bit slack so I feel guilty which makes me feel like I can't delay to make a point, but OTOH also I'm likely to be snappy/irritable.
But secondly - I think when they get stuck on asking for something repeatedly, even if it is something you've said a clear no to, they're actually telling you something else, like they're bored or having trouble moving on to the next activity. (Yes I know this sounds like a wanky social media parenting thing - bear with me). So I think it can help to strengthen the behaviour you want (ie, for them to move TF on) either by looking at a longer-term plan for how they can get the thing that they want - if it's something like "go swimming" then find a time when it does work and stick it on the calendar so they see it can happen. If it's something like "go to Legoland" and that's not on the cards then look at a smaller, cheaper/more local attraction which might have some of the lure of Legoland. If it's something you don't want to spend money on, consider making a saving up chart so that they can make progress with their own money towards it, maybe even a reward chart to reinforce some other behaviour.
And/or give them some positive attention in a different way and see if you can suggest something for them to do or something to do together just to interrupt the pattern and redirect their attention without it being a big back and forth conflict, which can be very attractive to some children (esp in ADHD) as well and again is just reinforcing the behaviour you don't want if you engage in the conflict or let yourself get annoyed by it.