Explanation of the error:

This Salesforce validation or deployment error that Gearset is surfacing may indicate that:

You're trying to deploy a component from your source environment with an API name X, while there is already another component in the target environment that has the same API name X taken.

This is when you're likely to come across this error:

There is more than one instance identifier with the same name: <xxx>

In this example <xxx> refers to the component name that has been identified as the component that already has the API name X taken in your target environment.

So it is not possible to deploy another component with the same API name.

We have seen this error occur during a deployment to a Salesforce org, specifically in a scenario where a Git branch was the source.

Resolution:

The resolution is to clone the component you're trying to deploy (this needs to be done in your source environment), give that component a different API name, and then deploy it to the target.

For more guidance on how to resolve the error, check:
Salesforce StackExchange forum.

Disclaimer: This error is returned by Salesforce directly, rather than Gearset. Even so, we offer guidance based on our combined experience with the Metadata API. Where possible, we try to help guide you to fix or avoid this error. In the case that this isn't possible, we may need to direct you to Salesforce support for further clarification.