dcast.data.table error message is cryptic when forced to cast duplicate rows without sufficient aggregation

The current error message is this:

Error in dcast.data.table(test.table, as.formula(paste(class.col, "+",  : 
  retFirst must be integer vector the same length as nrow(i)

A replacement candidate recommended by Arun is this:

Error: fun.aggregate should return, for each unique group (from formula's LHS), 
  a length 1 vector, but returns length=2 for a group.

This SO thread has a reproducible example and background describing the issue.