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Kara: I mean, CIark seems Iike a reaIIy nice guy
and everything...but I can't heIp but feeIing
he's hiding something from me.
She keeps feeling...?

Lex: Heh. As his former best friend,
I know the feeIing aII too weII. Very well?
Kara: See, (Does 'see' mean here you see what I mean?
or is it just 'and' as in And he didn't mention that?)
CIark didn't even mention
that you guys were friends.
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IMO "I can't help but feeling" is incorrect grammatically. It should be either "I can't help but feel" or "I can't help feeling." Either means "despite myself."

Although I love to eat veal, I can't help but feel sad when I think about the way calves are killed.

"All too well" means, literally, too well - you wish you didn't know whatever it is quite that well.

I understand how to live on a budget all too well. My husband was out of work for eight months last year, and we were almost evicted from our apartment.

"See" - here, I think it means "look at this example." Or in this particular case, "well look, what you just now said is an example of what I mean!"
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Thanks, Delmobile!