Mark
Based on the two answers you have already received, I would suggest you contact a LOCAL Realtor to have your questioned answered specific to your state and the laws and common practices there. In my market in Ohio if a property is listed in the MLS and showing PENDING as the status, it has a contract that has been signed by both the buyer and the seller and may or may not be subject to contingencies (financing, inspections, etc.) Until that contract expires, the contingencies are removed or the contract is mutually released, the seller would be opening themselves up for legal action by the initial buyer if they attempted to accept any further offers on the property aside from a SECONDARY offer, which you could put in to place and many Realtors and sellers are very willing to do that in today's uncertain mortgage market.
If a property is listed as CONTINGENT in our local MLS, there is a negotiated contract on the property that is subject to some other contingency...possibly subject to a home sale with a bump clause (if another offer comes in the first offer has so much time to proceed with their contract or it is bumped and the new offer takes over primary position) or it could be waiting on signatures from sellers in a verbally agreed upon contract...like an out of state bank or loss mitigation dept.
It all comes dowwn to the local terminology and common practices as they apply to the real estate laws in your area.....hope that helps.

Yes, you can always submit an offer. Your Realtor will send the offer to the listing agent who in tern is required to submit the offer to the seller. The seller will then decide what to do with your offer.If the house is pending it has a contract on it right now, but that offer probably still has some contingencies on it, such as a mortgage approval etc..

You need to insist that the seller signs your offer either as accepted or rejected, but that they DO SIGN IT! You want to be sure that the seller is the one who makes the decision. Your offer should be submitted with a cancellation of prior contract clause initialled by you.

Then the whole situation is in the hands of the seller and their representatives (attorney and Realtor) as to how ,and if the first contract can be cancelled and even if they want to.

If the house has an accepted offer you can put in another offer. If the house is under contract you cannot.
In todays market the seller is not always accepting the highest offer-the terms of the sale are as important.
i.e. - Does the buyer have anything to sell? How much money do they have for downpayment? Do they have a good credit score and will be able to get a mortgage with no problem?