Eh, why on earth would you feel bad for getting second hand? They don't care, clothes are clothes.
It will cost a fortune if you're getting it all new, especially from places like Next etc, and if you're waiting until they have outgrown things and then buying an entire set of clothing in the next size.
I do think you probably have higher costs with twins as the eldest though because you need to buy double in that size - however you should have double the hand me downs which means twice as likely not to be worn out! So might balance out a bit for the younger one, though I still think twins as the eldest is likely to be the issue.
I used to keep an eye out for bits I liked in sales, second hand, reduced etc and buy it in the next size up to keep on hand, so that I had a stock ready to go and then when I notice something isn't fitting quite right, I can be on the lookout for trousers or whatever in that size. Usually then I only need to buy a couple of pairs at once to fill in gaps, and so then I specifically look for multipacks or 3 for 2 etc, and tend to go for more basic items to keep costs lower. Whereas when I'm looking second hand, in sales etc I allow myself to buy nicer bits. Marks and Spencer is great for multipacks. Next often have multibuys. Supermarkets sometimes do surprisingly nice bits, and are perfect for basics like underwear. I have found Primark totally deceptive because they are cheap but nasty quality. But they are alright for things like jeans, pyjamas and sometimes hoodies.
Also keep it simple. I think this is easier with boys' clothes than girls' - but DS literally only wears shorts/trousers, then short sleeved t-shirts and jumpers/hoodies. I don't buy things like shirts or other complicated combinations which can really only be worn together. He did have some dungarees when he was younger, but they can go over a long sleeved t-shirt (which he used to wear when younger) or bodysuit for babies/toddlers which is fine. If I had a girl I'd probably stick to a combination of leggings/tights and then tops and dresses, maybe a couple of pairs of shorts or a skirt - basically anything which can be mixed and matched rather than having lots of complicated outfits which need to be assembled, because I think that leads to needing more clothes as things are in the wash or whatever.
Lastly budgeting helps - I used to have a ballpark figure of paying max £4 for a t-shirt and max £8 for a pair of trousers or a jumper. £12 for a pair of pyjamas although I'd try to get them cheaper (I hate overspending on sleepwear). I'd only go over that figure if I saw something I really loved. And usually second hand you can get things for much cheaper than that. I'm not sure if £4 and £8 is realistic any more as DS is nearly 10 now and I'm not familiar with UK prices since we moved abroad, but find out your limit and keep it in your head when you're hunting.
Brand new character or branded stuff is for Christmas/birthday presents where I'll often buy something which is needed anyway but instead of spending my usual on it I'll look a bit harder and splash out on something nicer. That way he gets a few nice things inamongst my bargain hunting. It seems to be working out as a strategy. And often he will gravitate towards one of the cheapest items I had picked up as some kind of afterthought, anyway!