This is exactly what I have tried before you had posted it
Sadly PGNExtract can't do it either
The -b arg is just for game length boundaries not for length
of move sequences to be compared as double
It seems no tool exists, which can do what I want to do
and moreover I believe now the task is not trivial at all.
The tool must be able to sort all games first by ECO or similar
and then compare each game as a sequence with given length
to the next game.
Without sorting it needs to compare each game against all
games each time and that would slow it down a lot.
I have found a manual solution so far, with the help of CB8,
using a kind of pattern recognition.
First I sorted all games by ECO then I displayed all games
only as moves list and looked at it around the region of the move
number I wanted to have as *double margin* e.g. move 15.
I noticed that the Human eye is very good in recognizing patterns
between all the move lists, as identical sequences in more lines
following another create a pattern in the sea of characters, because
of identical spaces etc...
This method is very exhausting for the eyes though, but I needed
just 5 minutes anyway to find 241 games which had at least
one sequence up to 15 moves twice or more times.
Guenther