Commercial Fixed Wing
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (mon) 8
Communication Breakdown; Workload
Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Environment - Non Weather Related; Staffing; Human Factors
Aircraft Y was at 2;000 feet MSL; and issued a climb to 3;000 feet to avoid converging VFR traffic who I was not talking to (squawking 1200) observed at approximately 1;700 feet MSL. Sector complexity was high due to IFR/marginal VFR conditions as well as an increase in usual volume we have seen over the past months. I was working 2 different frequencies as well. When I issued a climb to 3;000 feet to Aircraft Y; he read back 3;000 feet; and then I moved on to other priorities. The next time I observed Aircraft Y's altitude; I observed 3;400 and told him to stop his climb and issued a turn away from conflicting IFR traffic (Aircraft X at 040). I also turned Aircraft X away from Aircraft Y and issued traffic. I did not issue a Brasher warning.This was the busiest session; both traffic and complexity wise I have worked in many months due to decreased traffic volume due to COVID-19. I feel like if I had scanned better I would have caught Aircraft Y's altitude bust before I did. If my workload increases to this level in the near future; I feel asking for a split from the departure sector; or even calling someone over to act as a coordinator; or a '2nd set of eyes' when I feel task saturation is causing my scan to break down would be beneficial. I feel that both of these actions could prevent this from occurring again.Another important take away from this event in preventing [a] re-occurrence is to not have an expectation bias assuming a pilot will comply with your instructions; even if he reads it back correctly. A constant scan is absolutely critical at all times; (busy or slow) to ensure pilots are doing exactly what you told them and to prevent separation/deviation errors due to complacency and/or expectation bias.
TRACON Controller reported an aircraft overshot an altitude clearance resulting in a conflict.