Marrying without her father’s knowledge

I am looking at a lengthy engagement period before my fiance and I am able to financially marry and live together. We would both like to have a nikkah during our engagement so that we are not sinning when we are intimate with each other. However, both of our parents are against this and want us to wait until our marriage reception which is in a year or so. Is it allowed for us to get a nikkah in secret? My second question is, is it ok for us to repeat the nikkah ceremony again during our reception (since our parents won't know of our first nikkah)? Is a double nikkah permissible to the same person? Your help is much appreciated.