I agree Princess of Wales should re-wear a few dresses. She has some lovely ones, that she's only worn once. Max's wedding day dress is a re-wear and the outfit is so lovely it's great to have it to admire again.
I had assumed that she would rewear one of her evening dresses, and had predicted it would be the green sequinned Jenny Packham that’s very similar to what she actually ended up wearing. But looking back though her wardrobe of evening gowns, actually there are very few that would be suitable for wearing in a Muslim country. Most are sleeveless/low cut/backless etc which rules them out pretty much straight away.
These three would have worked, but you’d have a tough time reinforcing the lace/gauze bodices on the light blue and black dresses to wear the Royal Family orders and stars she needed to anchor (there was another light blue dress that had an even more gauzy overlay, which I discounted). The black probably isn’t a wedding gala colour either, which leaves the bright blue dress, which isn’t really fancy enough without the shawl, I think?
And I was struggling to think of another long-sleeved, floor length dress that she’s worn that would work for the ceremony itself? There’s a long cream Roland Mouret dress she wore 10 years ago or so, but it’s a bit dull, and had a big slit up the font!