Depends. It happened with 2 contracts so far: Hawk and Crow. What was allowed was the owners to keep the custom versions while the cannon version existed in parallel. Basically, Ryujin for example has a Crow contract but has the ability to have 10 custom summons for it while still being able to use the cannon crows.
When it comes to techniques themselves, depends. If it really is the same, an understanding must be found with the member. Cannon always triumphs customs. But we never do things to let members get the short side of the stick. Customs, and I know this personally, is something that takes tons of time, efforts, etc to make. We don't toy with it unless some really big reason exists. As far as I know, it hasn't ever happened.
Now, if you speak about people trying to submit them...well, then its another thing. Because every time i check customs, there is at least one or two submissions of cannons. Worded differently, clever named. But cannons none the less.