Okay, this is a dangerous situation, and you took the correct initial steps, but capping the extra feed isn't the final solution. Here's a breakdown of the situation and what needs to happen

Understanding What Happened
Double Feeding

This is a serious code violation and a significant safety hazard. Connecting two separate circuits to one receptacle is dangerous because:

Overload

It can overload the circuit breakers, potentially causing a fire.

Voltage Differences

Even if both are supposed to be on the same voltage and phase, slight differences in voltage can cause circulating currents and overheating. As you correctly pointed out, if they were on different phases of a 240V system, you would have had 240V at the receptacle, which could have fried anything plugged in, or worse, shocked someone.

Backfeeding

One circuit could potentially backfeed into the other if one circuit's breaker is off. This can energize wires that are assumed to be dead.

GFCI Failure

The GFCI receptacle, in your case, failed in a dangerous way. While newer ones are designed to fail open, older ones can indeed fail closed, offering no GFCI protection while still passing power.

Why Capping Isn't Enough

While capping the extra feed is good as a temporary measure to prevent shorts, it doesn't address the underlying problem. Someone could easily come along later and reconnect it, unknowingly creating the same dangerous situation. More importantly, that circuit is still live with an open end somewhere and that is a fire and shock hazard.

What You Need to Do1. Identify the Circuits

You need to definitively trace both circuits back to their respective circuit breakers in the electrical panel. Label each wire clearly. This may require systematically turning off breakers and using a continuity tester.2.

Determine Why There Are Two Circuits Feeding One Location

This is the most crucial step. Why was this done in the first place? Possible reasons:

Ignorance/Mistake

The most common reason. Someone simply didn't know what they were doing.

Overloaded Circuit

Someone might have tried to alleviate an overloaded circuit by tapping into another one. This is a sign of a bigger problem (insufficient circuits for the load).

Extending a Circuit

A sloppy way of extending a circuit.3.

Correct the Wiring

This depends on why the second circuit was run:

If it was a mistake

Remove the extra circuit entirely. Disconnect it at the receptacle AND at the breaker panel. Cap the wires securely in the panel and label them "UNUSED".

If it was to alleviate an overload

This indicates the original circuit is overloaded.

Redistribute Loads

Move some of the appliances or devices plugged into that circuit to other existing circuits.

Install a New Circuit

If redistribution isn't enough, install a completely new, dedicated circuit to handle the load. This is the safest and most proper solution.



If it was to extend a circuit

Remove the extension. If more receptacles are needed, extend the circuit properly in a daisy chain manner.4.

Replace the GFCI

Now that the wiring is corrected, install a new, properly functioning GFCI receptacle. Test it thoroughly to ensure it trips when the test button is pressed.5.

Warn the Customer

Explain the dangers of double-feeding circuits. Emphasize that they should never attempt electrical work themselves unless they are qualified. Recommend having a full electrical inspection to ensure the rest of the house is wired correctly. This kind of amateur wiring often leads to other hidden hazards.

Important Considerations

Code Violations

Double-feeding circuits is a serious code violation. Depending on your local regulations, this might require a permit and inspection.

Liability

As the electrician, you are now aware of a dangerous condition. You have a responsibility to correct it properly and document the work you did. Failure to do so could expose you to significant liability if something goes wrong later.

Professional Help

If you are uncomfortable with any of these steps, hire a qualified electrician. This is not a situation to take lightly.

In SummaryYou've identified a significant safety hazard. Capping the wire is a good start, but it's not a fix. You need to identify the purpose of the second feed, correct the wiring, and ensure the customer understands the dangers involved. This might require a more extensive electrical inspection of the property. Flag for review