Air carrier Mechanic reported incorrect maintenance procedures preformed during the repair of an aircraft fumes discrepancy.

Narrative

On Date Aircraft X was taken out of service for a SOF (smell of fumes) report/log entry. Multiple shifts and mechanics were assigned to troubleshoot. I was not on shift for that day. On Date1; Aircraft X was still assigned to be worked on and was tasked to it along with other mechanics (numbering more than 5). Turnover from prior shifts were poor; but some mechanics were tasked on it the day prior and continued on with the shift I was working. All jobcards pertaining to the SOF event; when I was tasked that day had multiple duplicates out in use. [Job Card] especially had more than 3 duplicates. All partially written on with info. Multiple mechanics took separate sections and used these cards for reference. None; however; were stamped. When I was tasked to do the decontamination card; I had completed it early in the shift under relay that a Phase 1 decontam was only needed. Before even signing off on this aircraft; I insisted on engine runs to make sure there was no odor. When we did our engine runs; I immediately smelled fumes and started over on the decontamination card and we proceeded to go through a Phase 2 decontamination. We spent almost all 2nd shift to complete. The work card was stamped as each step of the decontam was done. All steps were completed. Jobcards were left in our office for turn in which wasn't done until the next morning. The workcards that were entered and handed in at end of shift were not the correct cards and should have been discarded. The card that was handed in was not the correct and final workcard that was complied with. also; the final workcard that was handed in was also a reference card and not the correct card to be turned in. All steps in the decontamination card under Phase 2 were complied with. The incorrect workcards were handed in at final. Poor shift turnover. Multiple workcard copies for reference that were not properly discarded and handed in as final working copies without verification. Multiple crews at shift change with lack of direction and knowledge of prior work accomplished signed for. Mechanic's unwillingness to sign off on their own work they accomplished.More thorough turnover with documentation even if a mechanic is continuing on next shift. Each shift; regardless if a mechanic is staying on; by shift's end should have a written turnover on a non-routine form. Turn in all paperwork when task is complete so leadership can properly verify work completed. Leadership needs to better familiarize with assigned tasks and workcards to properly account for steps taken to correct errors and to identify conflicts with work being performed and steps being signed off on.