I've never figured this one out myself.I deal with about 110 toilets in an apartment building and for an unknown reason,sometimes I get two flushes for one.Since I have a wide variety of other "situations", I've never really pursued this phenomenon.So long as you're not using alot of extra water and the rest of the toilet works,I'd take the bonus flush.

Don't know, but if I was betting, I'd bet it's a design flaw in the toilet. Matches, are all those the same brand? Just curious.

I didn't have time to get into this yesterday but I deal with some eccentric, high rent paying clients who can be annoyed by such things.Several plumbers have given me the "I don't know who cares" look when I asked this question.In 13yrs. I have found that in my case,a northeast wind across my four story roof must do something to the venting but doesn't effect all toilets nor are all of the same toilets effected the next time.
I do have one or two toilets which double flush all the time no matter what.Like I said, if all else is well,who cares?But I'd like to know what that noise is that follows the second flush,you know..burple,burple,burple...what is that?By the way,most of them are Gerber rear flush toilets.

[This message has been edited by Matches (edited August 30, 2001).]

nammc,
just thinking about your information. You said if you slow down the water flow, the problem doesn't occur. I'm wondering if the problem would be solved by reducing the flow of water that goes down the little tube (don't know the name of it) that's attached to the intake valve and injects water down the center tube into the bowl.
If the bowl fills too fast, relative to the flapper closing, could that be causing the problem?

Could it be that the chain from the flush lever to the flapper is somehow getting tangled on the float arm? As the toilet fills after the first flush, the rising float would pull up the flapper causing a second flush. The water rushing over the float arm during the second flush would dislodge the chain to prevent a third flush.

Somehow the flapper has to be pulled up for the second flush after the tank is filled or partially filled after the first flush, right?

It would take the right combination of extra chain and proximity of the flush lever and float arm, but I guess it could happen, particularly if the flush lever is pushed real hard or held down for several seconds until the float falls below the level of the chain on the flush lever, thereby letting the chain get tangled on the float. Does it happen when the flush lever is pushed lightly and quickly?

I guess that the fix would be changing the length of the chain and/or bending the float away from the flush lever and chain.

rpxlpx,
I compared the two toilet (one upstair and one down in the basement) and found the toilet that having problem with flushing twice (upstair one), the flapper does not closes after the 1st flush. It closes a few seconds later, which means that on the 1st flush ,all of the water did not exit the tank and go into the bowl. The level of water left in the tank after the 1st flush prevent the flapper from closing right away. When I flush the one in the basement, the flapper closed as soon as the water exit the bowl. Hope this info would help you solve the problem. Thanks for your replies.

hmmm, so the question is one of 2:
1) Why doesn't the tank empty completely on the first flush?
OR...
2) Why does a second flush occur before the tank can empty and drop the flapper?
I'm still back where I started. I'm thinking the bowl is filling to the point of flushing a second time before the tank has emptied once. That still makes me suspect the flow from that little tube. What happens if you redirect it to flow into the main part of the tank?

rpxlpx,
When I redirect the flow, it still have the same problem. I suspect the problem is in the bowl, where I see a lot of bubbles pushing back into the tank which I think is preventing all the water from exiting the tank into the bowl when it flushes.

That was my best shot. Sorry I couldn't be of more help. If you find out the cause, maybe you could let others know by posting here.