Yes, we have no rules regarding sending the same book through the beta service multiple times and there are reasons why some authors may want to do this. 

For example, if you send it through and there are multiple or major issues discovered that requires a lot of rewrites to your story, especially if it significantly changes your plot or character development, you may want to get another round of feedback once all of those changes are made.

It’s important to note, however, that Hidden Gems cannot guarantee that you will get the same (or different) readers signing up for a book that they already beta read.  However, you can make this more likely by using the request/block checkboxes on the My Beta Readers page of your dashboard.

For instance, ahead of your book running the second time, you can request that the readers that read it the first time be selected if they sign up again, or alternatively you could block those readers from signing up again.  Just remember that if your intention is to just request/block them for that one particular book that you undo those changes after that book is sent to readers, otherwise those readers will be requested/blocked from all your future books until those flags are removed.