I had a similar situation during the Second draw, I raised a CSE in Feb asking why received in FSW. Then I received the response from CIC in 4 days which states that "The order of precedence will be followed like FSW, CEC, PNP, FSTP if you qualify for more than one in the sequence, However, you are under no obligation to accept this invitation and you may be invited for the next one in the order".
So after few days they reviewed my invitation and found that I am ineligible for FSW coz of the fact that I do not have the ECA done but due to the system error, I received the invitation. So, I received an email from CIC asking your invitation is sent out of system error however, we do see that you qualify under CEC and you may receive the Invitation shortly. After 10 mins I got the new invitation under CEC, thru which I am a PR now.
As our friends said you may decline this and may receive under CEC. But I would suggest you to send a case specific enquiry and ask them will you get an invitation under CEC if you decline the FSW one or do you have to wait until next draw and should cross the threshold to receive the new ITA under CEC? This is a big decision if you decline now and if you are not able to make over the threshold in the next draws, then you will regret for the decision. So, send a CSE and check with them.