From #1067, there is a bit of ambiguity in the handling of config file
with vector input and multiple consecutive parameters. For example
```toml
option1=[3,4,5]
option1=[4,5,6]
```
Currently this is handled as if it were
```toml
option1=[3,4,5,4,5,6]
```
But this could be confusing in the case where the input was referring to
a vector of vectors.
This PR adds a separator in the sequence to separate the vector so they
are two vectors of 3 elements each.
Will need to verify if this change has other side effects. It is a
pretty unusual situation.
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