In rare cases, duplucated variable names may appear in the model.
For example, when the first variable is named 'V1' and is a factor, and
there is a variable called 'V10' and/or 'V11' and/or 'V12' (etc), which
is/are numeric. For the binary factor V1, dummy contrast variables were
created to fit the model, called 'V10', 'V11', 'V12' (etc). As should be
clear from this example, this yields replicated variable names, which may
yield errors or incorrect results. Users should avoid this situation by
renaming the variables prior to the analysis.