Question on basic cloning theory

[rocky]

Hi 

Can anyone tell me:
Is there only one plasmid goes into single competent cell?

If yes, why?  why not 2 or more plasmids?

If no, how can I make sure I get a unique clone from the single
bacterial colony?

(  Note: I carry out an PCR for the target using mini-prep rised from
single colony but find 2 products with ~100 bp differences. Not sure
the 2nd product is non-specific or not since sequence analysis not yet
done! )
Rocky
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