Scarleteen

Hi.

Is it safe for someone to keep ejaculating inside the vagina at one time. For example, if penetrative sex kept going on and a man kept ejaculating inside would it be okay?

It depends on what you mean by safe! If this is a situation where a condom is being used, for example, it's important to withdraw from a partner after an initial ejaculation, before the erection is lost; staying inside a partner past that point could cause the condom to be less effective.
Other than that, though, there isn't a significant increase in risk of pregnancy or STI transmission if someone ejaculates in their partner once vs. twice at a time.

So, if a condom is not used, besides STI transmission would it still be okay for the vagina?

If a condom is not used, there are both pregnancy and STI risks regardless of if the person ejaculates once or twice.

More on those here:
STI Risk Assessment: The Cliff's Notes
The Pregnancy Panic Companion