It appears that this bug was introduced in Maple 9.5 as part of the
automatic-simplification project (see
?updates,Maple9_5,compatibility).
Expressions of the form x-x are no longer automatically reduced to 0
since x-x is not always 0; as is the case when x is a matrix.

The handling of SUMs containing common sub-expressions (including
the constant multiplier) are being corrupted after the first-pass of
simplification before evaluation occurs.  The first usage of said
expression works perfectly.  When the second instance the same
expression occurs, the partial result is used, leading to surprising
results.  

We have isolated and fixed the problem, and hope to make a patch
available asap.  I'll post instructions on this thread when it is
ready for download.

Regards,

Paul DeMarco
Manager of Kernel Development, Maplesoft