Duplicate lines in CSV output for CS FMU

[wojciech-graj]

Description

There are 2 identical lines written to the CSV output for each timestamp when simulating a cosimulation FMU. If the FMU is built as a model exchange FMU, this does not occur.

Steps to reproduce the behavior

Create the files BouncingBall.mo and build.mos, then run the shell commands, and inspect res.csv.

BouncingBall.mo

model BouncingBall "The 'classic' bouncing ball model"
  type Height=Real(unit="m");
  type Velocity=Real(unit="m/s");
  parameter Real e=0.8 "Coefficient of restitution";
  parameter Height h0=1.0 "Initial height";
  Height h "Height";
  Velocity v(start=0.0, fixed=true) "Velocity";
initial equation
  h = h0;
equation
  v = der(h);
  der(v) = -9.81;
  when h<0 then
    reinit(v, -e*pre(v));
  end when;
end BouncingBall;

build.mos

loadFile("BouncingBall.mo");
buildModelFMU(BouncingBall, version="2.0", fmuType="cs");

Shell commands

omc build.mos
OMSimulator --resultFile=res.csv BouncingBall.fmu

Output (res.csv)

time,h,v,der(h),der(v)
0, 1, 0, 0, -9.81
0.002, 1, -0.01962, -0.01962, -9.81
0.002, 1, -0.01962, -0.01962, -9.81
0.004, 0.99996076, -0.03924, -0.03924, -9.81
0.004, 0.99996076, -0.03924, -0.03924, -9.81
0.006, 0.99988228, -0.05886, -0.05886, -9.81
0.006, 0.99988228, -0.05886, -0.05886, -9.81
0.008, 0.99976456, -0.07848, -0.07848, -9.81
0.008, 0.99976456, -0.07848, -0.07848, -9.81

Expected behavior

I expect the output file to not have 2 entries for each timestamp.

Screenshots

N/A

Version and OS

• Version: OMSimulator 2.1.3~9-ge3a3aeb4-linux-notlm-debug
• OMC Version: OpenModelica 1.26.1
• OS: Debian GNU/Linux 13 (trixie)

Extra details

As far as I can tell this is an issue with OMSimulator and not the FMU itself, because fmusim from modelica/Reference-FMUs doesn't exhibit this behavior.