While debugging a transformation I built, I found that my stream lookup, when set to have multiple copies was not finding matches for an incoming record that was there. I have 3 incoming records (all with the same incoming value). It found one match, but used the default value for the other two, even though they all had the same value. When I set the step back to one copy, it then found matches for all of them.
Solution was to set the data movement from distribution to copy. This is by design.
User was always under the impression that this was handled in the background when you created multiple copies. But this is not true for steps that act with info steps.
Could be adressed by a BIG warning to the user.