Homeowner added a garage circuit himself.

No, this is not fine and must be corrected.

Code Violation: NEC 310.15(B)(1) requires conductors to be sized appropriately for the overcurrent device. More critically, NEC 110.14(D) and the breaker manufacturer's listing (Square D Homeline) prohibit connecting two conductors under a single screw unless the terminal is specifically rated for dual-lug operation — which standard Homeline breakers are not.

Safety Risk:

• Loose Connection: Even if tightened, two wires under one screw can create an unreliable connection, leading to arcing, overheating, or fire.
• Insulation Damage: The wires may rub against each other or the screw, degrading insulation over time.
• Breaker Trip Issues: Uneven current sharing can cause premature wear or failure to trip under overload.

Immediate Action Required:

1. Turn off the breaker.
2. Install a new 20A breaker (if space allows) or use a listed tandem/dual-slot breaker compatible with Homeline panels.
3. If no space, consult a licensed electrician to add a subpanel or rearrange circuits safely.

Do not leave as-is. This setup is a fire hazard and a code violation. A licensed electrician should evaluate the panel layout and correct the installation properly.

No. Remove it immediately.

Code Violations (NEC 2023):

• 110.14(A): Terminals for one conductor only unless listed for multiple. Standard Homeline breakers (HOM) are not listed for two wires under one screw.
• 110.3(B): Equipment must be installed per listing/labeling.
• 310.10(H): Splices/terminations must not damage conductors. Double-tapping a single-hole lug crushes strands, creates high resistance, and causes overheating/fire.

Why "Working for a Year" Means Nothing: Heat damage is cumulative and hidden. The connection degrades over thermal cycles. It passes a "tug test" until it fails catastrophically.

Correct Fixes (Pick One):

1. Tandem Breaker (Best/cheapest): If panel label allows, replace the single 20A with a HOMT2020 (2-pole tandem) or HOM2020 (quad) to gain a dedicated slot. ~$15–$30.
2. Pigtail (If slot exists): If you have any open slot, move one circuit to a new breaker. Splice the two #12s + a 6" #12 pigtail with a listed connector (e.g., Wago 221, Ideal In-Sure, or tan wire nut). Land pigtail on breaker.
3. Double-Lug Breaker (Rare): Only if you find a Square D HOM2020DL (listed for 2 wires/pole). Hard to find; tandem is standard.

Do not use a "double-tap lug" adapter on the breaker screw—those are for panel lugs, not breaker terminals.

Bottom line: This is a latent fire hazard. Fix it properly today.