For a lot of these outfits, I'm not sure if she ever wore them again. I do know that there are alot of engagements that Diana carried out, that we will likely never have pictures of. For example usually once or twice a month she would receive an official from one of her charities or from one of her regiments. I would imagine that an official picture was taken, but they were rarely published. So we don't know what she wore for these engagements.
As for the 1981 Opening of Parliament gown, suprisingly enough, she wore it again at the next year's Parliament. At least it looks like the same dress.
The blue gown she wore to the CFDA awards was also wore for a portrait in late 1995 (she's sitting in front of a painting that was done of her, remember that one?)
The blue dress that she wore for her AIDS handshake in April 1987, she wore it before that on Sept. 24 1986 (picture below).
And as for the "dressing gown" style dress & the "Anorexia" dress, they were probably best left in the closet.
Photo: D. Levenson