There would have been no issue in signing a another contract provided the second agreement also have the same terms as the first subcontracting agreement. A sub contractor agreement specifies the terms of the relationship between the main contractor (yourself) and the subcontractor (Another party). It is required when a business does not supply all the goods or services for a project directly but engages a subcontractor to provide some of the goods or services.
We do have a Subcontracting Agreement on Zegal platform to know more read the article.
It would have been ok, but it seems they are adding new clauses there which are different as mentioned above. In this case, you can refuse to agree to the new contract and have a discussion with them to sort out the issue and make sure the terms and conditions are agreed upon by both the parties. Explain to them that there can't be two conflicting contracts for a same project, tell them you would have been happy to sign such a contract provided the terms and conditions were the same as the subcontracting before and they should have not endorsed or requested changes to the main contract when they had a chance and had more discussions around the first subcontracting agreement. If both parties agree they can sign an addendum to subcontracting agreement so as to agree to new/updated terms and conditions, if need be.