I’m getting confused on certain phrases and whether or not to use subjunctive

If I were to say “I don’t know if you know this but…”

Would it be “no sé si sepas esto pero” or “no sé si sabes esto pero”?

I get subjunctive is used when their is doubt or uncertainty but still not sure I’ve heard people say the phrases like the first.

Any clarification would be awesome!

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