For an RAD to be launched by the community, a topic must receive 60 signatures (or 300 in the case of an ERAD). You might have signed a petition that was calling for an RAD to be launched. Since that petition was successful the topic has now been brought to a decision making process. Your previous signature was used just to launch the process. Only your vote in the RAD process, counts towards the decision. 

In all likelihood, this error message appears because you have already submitted your vote. You have either voted previously or double clicked the submit button. The software registers each click as a different attempt. As we allow only one attempt per person it shows an error. 

Please contact the RAS if you have any doubt that your decision has been registered.

The voting link that you have received is a personalised link for you and can be used only once. If you send this to someone else, it will not allow them to vote (if you have voted already) or you will not be able to use it to vote(if they participated using the link before you have). 
If you want your friend to participate ask them to check their inbox for their own voting link. IF they have not received it, they can get in touch with the RAS.

We would like all participants to participate of their own will and register their own choice. We seek to ensure that one's choice reflects one's own informed decision and maintain the principle of one person, one vote. 

The Residents' Assembly Service (RAS) does not "represent" the RA. The RAS is a service body that organises the decision making and working group selection processes of the Residents' Assembly.