[R] Add test coverage for joins with duplicate columns

[HyukjinKwon]

Describe the enhancement requested

c273ea7 introduced a todo below:

arrow/r/tests/testthat/test-dplyr-join.R

Lines 191 to 192 in de6eb89

	# TODO: test duplicate col names
	# TODO: casting: int and float columns?

Component(s)

R