i have them in my storage in case my friends needs any
at one point i might just give them away because i'm starting to have too many in my storage and that space can be used better for other things ;n;
i used to keep them on the path outside my airport -- one group for my friend, another for my sister -- but after a while, they were 90% duplicates, so i just ended up selling them all the time unless it was a "rare" one i thought my sister might not have.
I have a bunch in storage just incase anyone needs them. I stopped collecting the common ones and only collect the rarer ones now but even then if im too far from a storage container they still might get tossed/sold .
I have a giveaway thread where I list my extra DIYs and recipes, and I keep the DIYs and recipes in my storage until I can give them away. Any duplicates get sold to Nooks because I had 4 rocking horse DIYs at one point, and it was pretty unlikely that I would manage to give away all of them. The giveaway has worked out pretty well so far because people are getting the game for the first time or creating new characters pretty often. 3 people have asked me for items in the past week, though that's a bit higher than usual.
Being an OG player, I remember before 2.0 that you couldn't store DIY recipes in your home's storage, so I just sold all the duplicates to Nook's, and still do if I decide to make my island rounds. Nowadays, I either trash or sell them as I have all the recipes in the game now. There's really no point in keeping duplicate recipes if I don't play with friends online.
However, I sometimes send a few duplicates to my second player villager's mailbox to have them learn the recipe. It comes in handy if I need to make something quickly for their house's interior.
I keep most of mine in storage, though I'm constantly close to reaching the cap for my storage, so recently I've tried selling some I don't really need or giving them to my second file. I try to keep them around to trade with friends (which we admittedly only do once in a blue moon skfkdf) or trade on the forums (which I keep putting off). I really should just clear them all out when I don't need them but I never know when that will be and never feel like I'm at that point.
They used to be all over my beaches. I used to sell the more common ones. Now that we can put them in storage they all end up there. I have been restarting in Dec/Jan my second island so always have a need for the extra. Every once in awhile I hold a giveaway for them.
All my DIYs are extra now, so any that I find I dump in the entrance to my island so that visitors can go through them. I haven't played ACNH in ages, so maybe that prevents the clutter; I only seem to log in for events, lately. This recent farewell to ACNL has rekindled my interest in New Leaf, so who knows what will happen to my island in a few years.
I used to keep them in my alt character’s house until the 2.0 update, now they’re in my storage! I keep them for trades, in case any of my friends need any diys, in case I ever get a second Switch and copy of the game, or in case I ever do decide to reset.
I have 3 islands, so I keep them and every so often check if my other islands require them. If not, I add them into my giveaway section until I need the storage then I'll need to sell them to Nooks.