Most of the time you receive this error when you change or modify your schema project wherein your schema project is referred inside the orchestration project.
The question which arise in your mind from where should I remove this resource. From GAC or from References or from resources.
I would suggest to remove the reference of schema project from the referencing project and then remove the assembly of referencing project from the BizTalk resources and if possible remove the assemlies from the GAC as well just be on the safer side. Start building and deploying the solution in the required dependability.
Cheers, Vishnu