As long as the Contracts are different Types or different Years, then you can attach multiple Contracts to a User.

For example if you have a Contract titled: School Name Boarding for 2016-2017 under the Boarding Type and then you have a Contract titled: School Name Payment for 2016-2017. These Contracts can both be attached to the User.

If you have a Contract Titled School Name Boarding for 2016-2017 and you also have a Contract titled: School Name Payment for 2016-2017 under the Contract Type: Boarding, then you cannot attach both of these contracts to that user because they are for the same Year and same Contract type.