It is very common. There's "Boston Rob" Mariano, who was on Survivor 4 times and The Amazing Race twice, and had his own personal reality show, chronicling his attempt to become a professional poker player.
The Amazing Race has also recycled contestants from Big Brother and other smaller reality shows.
Here's a 2013 article about contestant recycling on the Food Network.
It's so bad that there's now even
"Marriage Boot Camp: Reality Stars".
This article mentions that "There are two schools of thought as to why this happens: One is that producers believe that viewers like the people so much we want them to return. “Survivor All-Stars” and other shows that bring back champions to play again definitely have this in mind. And some series populate their casts with familiar faces, possibly in the hope that they’ll draw new viewers. “
I think some of these people just want to keep their 15 minutes of fame going, others are trying to leverage their appearances into a "real" television career (like Jillian Harris; after her stints on The Bachelor and the Bachelorette, she's now an interior designer and co-host of HGTV's Love it or List it, Too.).