Learn Spanish (Latin America): Él es invencible, digan lo que digan.

This is a reduplicative subjunctive clause, a common structure in Spanish used to express indifference. The formula is [verb in subjunctive] lo que [same verb in subjunctive]. It translates to phrases like no matter what or whatever. Here, digan lo que digan means whatever they say.

Who is the they implied by the form of digan?

The verb digan is in the third-person plural form, but it does not necessarily refer to a specific group of people. Like in English, Spanish often uses this form to refer to people in general or public opinion, similar to saying whatever people say.

Why do we use the subjunctive mood for digan here?

In these repetitive phrases, the subjunctive is used because the exact action is hypothetical, unknown, or deemed unimportant. The speaker is declaring that his invincibility is a fact regardless of any potential opinions or rumors that might exist.

Can I use this repetitive lo que structure with other verbs?

Yes, this is a very common and versatile formula. You can use it with almost any verb to express indifference. Everyday examples include pase lo que pase for whatever happens, hagas lo que hagas for whatever you do, and cueste lo que cueste for whatever it costs.