Date: 201706
Local time of day: 1201-1800
Location: EUG.Tower, OR
Flight conditions: VMC
Light: Daylight
Aircraft: Light Sport Aircraft
Operator: Personal
Operating under: Part 91
Flight plan: VFR
Mission: Personal
Flight phase: Taxi
ATC / advisory: Tower EUG
Second aircraft: King Air C90 E90
Reporter organization: Government
Reporter function: Ground; Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Communication Breakdown; Situational Awareness
Anomaly: Conflict Ground Conflict; Less Severe; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
Detector: Person Air Traffic Control
When detected: In-flight; Taxi
Result: Air Traffic Control Separated Traffic; Air Traffic Control Issued New Clearance; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Aircraft; Human Factors; Procedure
Primary problem: Human Factors

Narrative: I was working Local Control and Ground Control combined. Runway 34R was closed. There were lots of calls on both frequencies. Ground Control had clearances; flight-following; taxiing aircraft; and airport vehicles (the latter going to/from closed runway). Local control had moderate VFR traffic; made more complicated by closed runway.Aircraft Y was inbound on RNAV Runway 34L Y approach; to a full stop. Aircraft X was taxiing out to Runway 34L. I figured out later I had messed up his flight-following destination; and had to coordinate that with approach. He said Crescent Lake; I heard Crescent Lake; but thought Crescent City.  This didn't have an immediate effect on the event below but it did have a minor effect on my recovery when I was wondering why he turned to a right downwind instead of a left. That was how I figured out my mistake on the destination.Aircraft Y was about 4 miles out. Aircraft X calls up ready to go. I told Aircraft X to hold short of Runway 34L due to landing aircraft and an aircraft crossing over midfield. I believe I said an airborne aircraft. Aircraft X didn't read back the hold short instructions so I asked him to read them back; Aircraft X read back holding short; still not a correct read back; but he did say holding short.I had planned on launching Aircraft X. I still had 2 miles with Aircraft Y; who was doing 110 knots. I checked out the window on Aircraft Y; scanned the runway; and there's Aircraft X sitting on the runway; in position. I sent Aircraft Y around immediately; offsetting to the right; launched Aircraft X after telling him he was supposed to hold short of Runway 34L; NOT go onto the runway; coordinated the go-around with approach while keeping Aircraft Y in the VFR traffic pattern. Then I sequenced Aircraft Y to land. All the while; I was also dealing with requests for taxi and flight following requests; as well as aircraft vehicles.Ground Control probably should have been opened. It was closed for a short time due to flex shift. Pilot to clean his ears out? This isn't the first time I've had trouble with this pilot not listening to instructions. Aircraft X hold short of Runway 34L; landing traffic and traffic crossing midfield; airborne. Not sure how much clearer I could be.




Synopsis: A Tower Controller reported that an aircraft taxied onto the runway instead of holding short; while there was another aircraft on final.