When I write papers  I frequently find myself have one the follow another the.
A simple example is "the star is the the  brightest known in that quadrant".
A search for "the *the" is not good enough since you can have interlopers
such as blithe the or tithe the  and other such unlikely or wrong combinations.

The easiest to catch is when both the the's are on the same line. 

The hardest is when one the is on line but the other the
the is on the next line.
Even harder is if you have two the's on one line and another
pair split over the next line but all over the same two lines
such as: the the brigthest star is really the
the one we detected last year.