Change error for when module is imported twice

[puffnfresh]

I recently did this:

import Data.Date (Now(), now)
import Data.Maybe (maybe, Maybe(..))
import Data.Array (filter, modifyAt, (!!))
import Data.Function (on)
import Data.Date (Date(), toEpochMilliseconds)

And kept getting a strange error:

  Unknown type Now

Took me a long time to figure out that I had Data.Date imports twice, and I only figured it out as quickly as I did because I've run into it before. It would be more useful to error:

  Data.Date module has more than one unqualified import statement.

[garyb]

Related: #817