Hi,

In 2020 it worked properly, now I am going back to use it again and it gives me an error:

```
library(nlme)
model_chg <- gls(change ~ method * time,
correlation = corSymm(form = ~as.numeric(time)|patient_id),
weights = varIdent(form = ~ 1|time),
data = pd_diffs_sub,
method = "REML")
Error in Initialize.corSymm(X[[i]], ...) :
covariate must have unique values within groups for "corSymm" objects
```

As usual in R when it gives an error it gives an info that I completely don't understand and have got no clue what to do.

Please help if you can.

best