Would you still wear it?

I once wore a very pretty navy dress, with multicoloured spots to a wedding. I bought it in a really small boutique and thought nobody else would have the same dress. I was wrong. At the evening reception, I met another lady wearing the exact same dress. We looked at each other, and both commented on our good taste, and left it at that. My DH did say though, that the dress looked much better on me!
That doesn't answer your question though. Yes I would still wear the dress.

I would let the much younger family member know, so she has the chance to wear something different!

I would beg the younger family member to wear something different!

I would style it up with different accessories and a pashmina or jacket.

I too, would style it individually, especially one of those nice bolero things that you can pick up fairly cheaply or make quite quickly.

I think that I would let your younger relative know that you are going to wear the same dress as her, easier for somebody younger to find another dress ( although she may not think so.) Don't ask her not to wear it, just say you have found out that you will both be wearing it, and that you can't get another dress, but will try and style it differently with wrap/pashmina/jacket.Leave it at that.......she can then decide what to do.

Another vote here for letting the other lady know, hopefully she will find another dress!

Not sure it really matters in the grand scheme of things. It's a day for the bride and groom. Accessorise differently and congratulate your relative on her good taste. Enjoy the day.

I'd wear it. Many years ago I wore a very expensive Louis Feraud outfit to a wedding for which I had saved up for months - 13 guineas! Another older guest was wearing the same outfit. I found it hilarious - she didn't and went home and changed - oops ? I'm sure as suggested you can personalise your outfit and maybe nobody else will notice and if they do, so what!

It just shows what good taste she has!

Thanks for the advice. It is quite a statement dress so only a plain jacket would do - which is what I'd planned. What I might do is let her mother know and see if she can break the news and let me know what she plans to do. I wouldn't want to ask her outright to change her plans as I know how much she's forked out for it. But as much as I feel good in the dress, she is a very beautiful girl and I would feel very much the mutton if we were both to pitch up in the same thing!
Thank you kitty, yes, excellent taste

Good idea to let her mother know. I don't think for one minute that you would need to ask her outright to change her plans - as soon as she knows her plan will change!

Wear your dress.
if you change it sods law there is still again a chance someone will be wearing the same.

Forget you found out. It doesn't matter in the long run, and if you hadn't found out, you would have worn it happliy.

I don't think you would necessarily look like 'mutton' Crumble just because a younger person is wearing the same dress.
The best and most stylish clothes don't relate to a particular age group - they can be worn by anyone who they suit in terms of colour and whether they flatter your figure, regardless of age.

I agree with ygg. There's no need to let anyone know that you know.

You will probably be wearing completely different shoes from the young guest. If you feel you must disclose to her or her mum, find out what sort of jacket she will be wearing, and choose something different.
Have a lovely time and enjoy the dress

Chances are, that as she is much younger, she won't wear a jacket at all. Young people don't seem to feel the need these days. So, you'll both probably look different anyway.
I wouldn't be too worried, just go and look wonderful and no one will notice the other dress!

just wear it and feel good. it doesn't matter if she wears hers too, but yes you could titivate it a bit with different accessories. The little bolero which another poster mentioned sounds a good idea. I LOVE Boleros

I agree with those who say wear it, you obviously like the dress, feel good in it, so enjoy the day, which is after all about the bride, and all eyes will be on her. You might not find the opportunity to wear such a lovely dress for such a special occasion again.

I would ignore the fact that you know, what if you hadn't found out you,you would be looking forward to wearing your nice new dress to a family wedding.

If you still feel ok about wearing it then go ahead I don't think the younger lady will fell that bothered although I would be inclined to feel a bit mutton dressed as lamb if I had to be in photographs with her

I think it is unfair to ask the "younger" person to change what she wears. The notion that younger people can go out and buy another outfit is ridiculous. Statistically, we baby boomers have more disposable cash than younger people. I think you should discuss it with her as to who is going to wear "it". The dress can always be kept for another occasion for either of you.

If it was me I would find something else to wear....

My older sister and I both unwittingly turned up in identical dresses (albeit hers red and mine navy) for our nephew's wedding a couple of years ago. I thought it hilarious and was going to say something about our shared good taste, but she was horrified and spent the entire wedding avoiding me, even refusing to be in the photos. In fact seethed with rage for nearly a year afterwards, which I think was a little extreme. A friend said she was probably cross because I looked better in it than her.

So go head, wear the dress, enjoy yourself and don't worry about the younger one - or mention it to her mum as someone else has suggested.

Could you wear a longish jacket for the actual wedding? After a few drinks at the do afterwards neither of you will care!