This looks like a solid, reproducible bug. It’s been filed as Bug #2791.
It appears that repeating the column attempts to delete the subsequent columns.
If no columns remain, then R crashes.
UPDATE : Now fixed in v1.8.11. From NEWS :
Assigning to the same column twice in the same query is now an error rather than a crash in some circumstances; e.g., DT[,c(“B”,”B”):=NULL] (delete by reference the same column twice). Thanks to Ricardo (#2751) and matt_k (#2791) for reporting. Tests added.