Sure, if you want the house, write a back-up offer. Should I assume you have seen the house by this time? If not, view before you write the offer.
Many deals fall out of escrow, so there is a chance you will still get the house. Your offer doesn't have to be better than what they have now. If it falls out, the sellers will probably be amenable to dropping the price, just from discouragement.
I don't know about killing the existing deal if a better offer comes in, but sometimes buyers need extensions, changes of terms, demand improvements, and the sellers can opt out of the contract at that time, and they might if they have a bona fide back-up offer.