Error in `row.names<-.data.frame`

Hello everybody,

I am still working on my R analysis, and using the contrasts. Specifically, I was needing a rectangular contrasts and I was actually taught how to create it. But when I give a column name to my contrasts, this creates an error in the summary() function of my lmer() analysis.

The code for naming the column of my contrast is:


colnames(posmat) <- "pos_c1"

While the error I get in the summary() is


Error in `row.names<-.data.frame`(`*tmp*`, value = value) :
  duplicate 'row.names' are not allowed
In addition: Warning message:
non-unique value when setting 'row.names': ?Pos?

I have written a CrossValidated topic with a MWE: https://stats.stackexchange.com/questions/349399/r-error-in-row-names-data-frame

Is anybody here aware of how that happens and how to solve it?

Best
Luca