Fix call to `duplicated()`

[maurolepore]

Closes https://github.com/2DegreesInvesting/TiltDevProjectMGMT/issues/107

#45 fixed a bug in the article "Get started" and #57 fixed it in README.

It would be great to write a unit test to ensure the bug does never return.

# Good
duplicated(paste(company_name, postcode))

# Bad: The first argument of `duplicated()` is not `...` it is a single vector `x`
# See `?duplicated()`
duplicated(company_name, postcode)

One way to tackle this issue is to write a little helper function, use it in tests, then use it elsewhere (README, Get started).

duplicated_paste <- function(...) {
  duplicated(paste(...))
}

You may fist write the wrong implementation and design a test that fails. The challenge is to figure out which data to use. Then fix the implementation and confirm the test passes.