Merge multiple data tables with the same column names

I am trying to merge multiple data tables (obtained with fread from 5 csv files) to form a single data table. I get an error when I try to merge 5 data tables, but works fine when I merge only 4. MWE below:

# example data
DT1 <- data.table(x = letters[1:6], y = 10:15)
DT2 <- data.table(x = letters[1:6], y = 11:16)
DT3 <- data.table(x = letters[1:6], y = 12:17)
DT4 <- data.table(x = letters[1:6], y = 13:18)
DT5 <- data.table(x = letters[1:6], y = 14:19)

# this gives an error
Reduce(function(...) merge(..., all = TRUE, by = "x"), list(DT1, DT2, DT3, DT4, DT5))

Error in merge.data.table(..., all = TRUE, by = "x") : x has some duplicated column name(s): y.x,y.y. Please remove or rename the duplicate(s) and try again.

# whereas this works fine
Reduce(function(...) merge(..., all = TRUE, by = "x"), list(DT1, DT2, DT3, DT4))

    x y.x y.y y.x y.y 
 1: a  10  11  12  13 
 2: b  11  12  13  14 
 3: c  12  13  14  15 
 4: d  13  14  15  16 
 5: e  14  15  16  17 
 6: f  15  16  17  18

I have a workaround, where, if I change the 2nd column name for DT1:

setnames(DT1, "y", "new_y")

# this works now
Reduce(function(...) merge(..., all = TRUE, by = "x"), list(DT1, DT2, DT3, DT4, DT5))

Why does this happen, and is there any way to merge an arbitrary number of data tables with the same column names without changing any of the column names?

via Chebli Mohamed