Have not run into this in the past when voting from the client.    Voted on one proposal then tried another and got this message using the Vote-Many command.   Waited a good 15 minutes and still getting this message.

 "......" : {"result" : "time between votes is too soon - 198c2be3e156c2a.......6a5f66acf4bba1 - 1537\n"

Version info:
Dash Core version v0.12.0.56-7154cdf (64-bit)

How long do you have to wait?