chandler: At one point, this would've discovered a minor-ish bug in chrome. Maybe you know someone who might want to work on this?

http://www.viva64.com/en/d/0090/ has more information about that warning and points out some cases where it doesn't warn.

Nico asked me to add some samples that represent real bugs that would have been caught with such a check. Each of these bugs are in various open-source libraries

Mesa3D, filed as https://bugs.freedesktop.org/show_bug.cgi?id=37648
Original:
else if (width > 0 && height > 0 && height > 0) {

Fixed:
else if (width > 0 && height > 0 && depth > 0) {

libjingle, filed as http://crbug.com/84129
Original:
if (!has_audio && !has_audio) {

Fixed:
if (!has_audio && !has_video) {

Webkit, filed as http://crbug.com/84141
Original:
return cy().isRelative()
|| cy().isRelative()
|| r().isRelative()
|| fx().isRelative()
|| fy().isRelative();

Fixed:
return cx().isRelative()
|| cy().isRelative() <-- Fix
|| r().isRelative()
|| fx().isRelative()
|| fy().isRelative();

Also http://crbug.com/84140 , http://crbug.com/84139

Original:
if (x >= 0 && x < paintingData.width && x >= 0 && y < paintingData.height)

Fixed:
if (x >= 0 && x < paintingData.width && y >= 0 && y < paintingData.height)

skia, filed as http://crbug.com/84138
Original:
return kKeep_StencilOp == fFrontPassOp &&
kKeep_StencilOp == fBackPassOp &&
kKeep_StencilOp == fFrontFailOp &&
kKeep_StencilOp == fFrontFailOp &&
kAlways_StencilFunc == fFrontFunc &&
kAlways_StencilFunc == fBackFunc;

Fixed:
return kKeep_StencilOp == fFrontPassOp &&
kKeep_StencilOp == fBackPassOp &&
kKeep_StencilOp == fFrontFailOp &&
kKeep_StencilOp == fBackFailOp && <-- Fix
kAlways_StencilFunc == fFrontFunc &&
kAlways_StencilFunc == fBackFunc;

possible testcases
I don't think we can support the "cy().isRelative()" case without being to noisy.

clang's r153568 suggests that warning on identical subexpressions next to >= (and <=, <, >) is probably useful, too.

A related idea would be to warn about identical condition expressions in if-else statements:

if (cond)
doStuff();
else if (cond)
doOtherStuff();

*** Bug #9565 has been marked as a duplicate of this bug. ***

work-in-progress patch idea
Attaching work-in-progress patch for safekeeping in case I loose it or my hard drive fails.