If I don't, will the annual policy pay out for the 2019 holiday if something happens to me between now and October to prevent me travelling in 2019?
I think it should do, but in fact it would be better to phone the insurance company and check as they are notorious for wriggling out of paying out on the slightest pretext.
As for having two overlapping insurance policies, I have that as we have one through the bank which will not cover for medical conditions but would cover for lost baggage, delays etc and also take out another one which covers for specific medical conditions.
When DD had some equipment stolen when she was in Europe we put in a claim and half was paid out by her travel insurance and the other half paid out by our household insurance. The insurance companies seemed to sort that out between themselves.