Maybe we're witnessing the birth of a new word.
I don't think this is a calque. The English word is unusual, and the Catalan word would only be a candidate if rapididez appeared only or mostly in Catalan-speaking areas of Spain. Rotundidez is also rather unusual.
Note that more than a few of your Googled examples are from pages in Portuguese. The correct Port. word is also rapidez. This doesn't go against your question; it actually suggests to me that rapididez is a barbarism that just tends to develop from rapidez.
If we discard the possibility of mistakes and misprints and assume that the writers really wanted to say rapididez, it looks like the suffix -ez is being reanalyzed as -idez, maybe by analogy with -itud, -ismo and -icia, and the speakers have "decided" that rapidez is missing something. This would be weird because rapididez is clearly cacophonic, so much so that if there was such a word to begin with it would probably be eroded by haplology. But there you have it.