HAProxy logged two FIX messages together for one session it was responsible for routing. See log lines 2-4 below (extra newlines added to make reading slightly easier ...

units metal dimension 3 atom_style molecular boundary f f f read_data ${infile} group 1 type 1 region 1 sphere 0.0 0.0 0.0 15.0 variable dt equal 0.0005 pair_style lj/cut 25.0 pair_modify tail yes ...