The dress she wore to Pippa's wedding made it look like she has droopy breasts (which she doesn't). I can't believe someone would pick out such an unflattering dress for her.
Kate wears new clothes to weddings as well as rewears. Her outfit to Eugenie’s wedding was new and in a lovely bold shade. She also wore a new outfit to Meghan's which was very similar but not the same one she wore to Charlotte's christening:
whatkatewore.com/2018/05/21/official-wedding-photos-solve-mystery-of-kates-coat-color/
I think the colours for Meghan's wedding were pre-organised, they created a nice pastel pallette in the official photo - HM wore green, Doria wore a pistachio green, Camilla wore pink and Kate wore light yellow.
Some Kate wedding guests looks that were new outfits. The electric blue Issa dress is similar to her engagement one, but not the same.