Is there a shim or something that locks out the extra shift when you use a 9 speed 105 road shifter for a double setup? I have finally pulled together all of the parts for my wife's bike and I'm about to make her triple a double (swapping the bonty triple, Ult RD, 105 FD for an FC-4550 compact, ult short cage RD, and Tiagra FD). She has the last generation 105 shifters, that were used for both doubles and triples. When I do this, what will be the effect of having one extra "click" left in the shifters?

I know that by adjusting the limit screws on the FD I won't have to worry about over shifting, but what will happen if she forgets what ring she is in and down shifts once past the small ring? Will there just be a lot of cable slack that she will have to simply shift again to take out, or is there some little piece of hardware that will lock out that third shift?

put the left shifter into the small gear position. de-attach cable from FD, take up slack and reattach.

the shifter won't wind up any more cable than what's limited by the FD and you won't have a drooping cable when the cable is released.

+1 Basically you first adjust the inner limit screw to position the fd properly over double's small chainring.

Then set the shifter to shift between the granny and the middle ring positions but you use the new inner ring as the granny and the big ring as the former middle. That way the high limit screw keeps you from moving the shifter and fd any further and locks out the extra click.

Not sure what you mean by "last generation". The current 105 is also double/triple compatible but 10-speed.
As the others have said, you set up the shifters and derailleurs as you would on any other component set only the double's inner ring is now the first position. The first position on any set is with all of the cable out of the shifter. Follow all of the instructions found at https://www.parktool.com and do them is proper sequence. The derailleur's high limit screw should keep the shifter from over-shifting.

Al

Great responses all, that's exactly what I needed to know, I was thinking about it all backward. For some reason I had it in my head that I would need to start at the middle, not sure why. In any case, what you all have said makes perfect sense. When I posted last generation, I just meant before 105 went 10 speed. I wasn't sure if the new 105 went the way of Ultegra, having a different set of shifters of doubles and triples. Thanks again, and thanks for the link!

I have two bikes that use the same (Ultegra 9-speed) STI shifters. My tandem has an Ultegra triple crankset and my road bike has an FSA compact double crankset.

The shifters have 4 shift positions. On the tandem I have 1 granny gear position, 2 positions for the middle ring and 1 shift position for the big ring. On the compact double I get 2 shift positions for each chainring. I'm pretty sure that's the way that Shimano intended because I also have Flight Deck computers on both bikes and they always indicate the appropriate chainring.