How do you stop buying things you already kind of own?

I'm trying to buy less but I keep making the same mistake... I'll buy something thinking it fills a gap, then get home and realise I already have 3 things that are basically the same. I think my wardrobe is full of similar items across similar colour palettes and silhouettes, but I can't see it clearly when I'm standing in front of it. How do you actually audit what you own before deciding what you need? Is there a system that works?