From reading your question I would have to assume that you closed out the task order under which this project was designed prior to construction being completed.  It also appears that you closed out the entire contract. One this is not recommended as there are often design issues that come up during construction that the government has to go back to the designer.  This has to do with the integrity of the design.  It is usually difficult if not impossible to get another design firm to sign off and put their liability on the line for these changes. 

If indeed the task order and IDIQ contract are totally closed out, there is nothing in the FAR that allows you to go sole source to this contract without following the rules and procedures at 6.302, Circumstances permitting other than full and open competition and FAR subpart 36.6.  Depending on the dollar values FAR 5.202 may require you to synopsis your sole source intention or post as a public notice.