mtcars$wt2 <- mtcars$wt
m <- lm(mpg ~ wt + wt2, data = mtcars)
m
## 
## Call:
## lm(formula = mpg ~ wt + wt2, data = mtcars)
## 
## Coefficients:
## (Intercept)           wt          wt2  
##      37.285       -5.344           NA
prediction(m)
## Average prediction for 32 observations: 20.0906
## Warning message:
## In predict.lm(model, newdata = out[[i]], type = type, se.fit = TRUE,  :
##   prediction from a rank-deficient fit may be misleading
margins(m)
## Error in jacobian %*% vcov : non-conformable arguments
## In addition: There were 20 warnings (use warnings() to see them)