To want BIL to ask what we would actually like for birthdays?

Just tell him (diplomatically) what you'd like. Or drop some heavy hints. Or even tell him his gifts aren't always appropriate. Otherwise, accept what he gives with good grace, then pass them on to someone who'd like them.

Easy stop the presents.
Just buy yourself what you need.

We have the same issue with our childen's grand parents. It is so lovely that they buy them extra treats but it is normally things like toy cars or baby dolls that our children already have too many of and will therefore be doomed to go to Vinted/Charity so it feels like a waste of money. I totally understand that having to ask ahead of time takes the fun out of it, but are these things really 'gifts' when they do not benefit the receiver in anyway.

No I don’t ask anyone because I buy what’s in my budget and they can do what they want with it. My ds is into Lego, switch games , a sport and those are expensive things! I’m not going to tell anyone to get him those sorts of things, in the same way I wouldn’t want to be told as well. I wouldn’t personalise anything though.
most recently I am just doing vouchers. I don’t care that it’s impersonal, but it seems like children have everything these days so I’m just going to make my life easier and do vouchers

That’s how I was brought up to do gifts. The giver chooses a gift for the recipient, and buys something they’d think the birthday child would like. Part of the pleasure is deciding what to buy, rather than being given a shopping list.

However, a quick email like @42goingon14 suggests, or a broad conversation that child is into Barbie not Sindy prior to the birthday wouldn’t go amiss.

We aren't close to them and see them very rarely (maybe once a year) so it's not like they know what we or the children like or want and base it off that.

Well to be fair they seem to have a pretty good grasp of what you do like if duplicates are an ongoing problem, you obviously must like the items if you already have them.

Have you mentioned it to them at all?

"Thanks so much for DS's present, it was really kind of you. Unfortunately he already has the exact same thing! Perhaps it's worth pinging us in advance to check when you are choosing next time. Thanks again!"

Type of thing?

My parents do this. I need to just agree that we don't do gifts any more. I particularly hate personalised gifts made of plastic as it's so much more likely to end up in landfill.

I have in-laws like this, it drives me mad! So their reasoning is that they like to pick something of their choosing and vice-versa, they want us to choose something for their child of our own choice that we think they would like. Which is a nice idea, however, as you say many duplicate gifts have been bought on both sides and quite often what they choose my children is not what they would want. My 18 year old was bought a cross stitch kit - she has never expressed an interest in cross stitch and hates intricate crafts.

I would love to stop the gift buying however, there’s a significant age gap between our children (& we have 2 and they have 1) we feel we need to carry on with the gift giving so it’s fair. We have tried the let’s take them all on an outing when they were younger but it fell flat. Given it’s my husband’s side of the family, I’ve made it a him problem and he refuses to give cash or vouchers. I understand your frustration!

Some people like to put the thought in to choosing gifts themselves, but maybe ask them to check with you or your DH before buying to avoid duplicates given this has happened multiple times

Maybe I'm just a miserable sod but this is why I find gift giving such a ballache. We are all so materialistic now that we mostly have anything that we want already. Gift giving becomes a pain because you can't win. People seem to take it as a personal insult if you get it even a little bit wrong. Gift receiving is a ballache because you're pressured either into being very specific about something you want even though you might not want any more crap in your life or you're pressurised into acting elated because you've been gifted yet more crap to deal with...

If your children already have the gifts your BIL is giving then they probably already have everything they could want or need. Just smile at BIL and say thanks, give it away to somebody who actually needs it and massively slim down the number of people you're exchanging gifts with. It doesn't need to create resentment or a horribly entitled attitude towards your loved ones.

Yes, just say stop doing gifts, you barely see them, they are making an effort, if it’s not good enough, just put a stop to it.

Bizarre! What do you buy them?

Why don’t you just say let’s stop doing gifts or do them at Christmas only but use Elfster or something?