no train to transport - strange lengths?

[bobDoesThings]

I occasionally get a no train message for train lengths that make no sense to me, like length between 10 and 10 when the real length required is between 5 and 5. What am I missing?

[Optera]

You are probably sending control signal twice.

[bobDoesThings]

yep, that was it. doing cleanup and left a second combinator lurking.