#1872 Remove extra carriage returns when fixing files on Windows

autoload/ale/fix.vim

@@ -30,7 +30,14 @@ function! ale#fix#ApplyQueuedFixes() abort
             call winrestview(l:save)
         endif
 
-        call setline(1, l:data.output)
+        " If the file is in DOS mode, we have to remove carriage returns from
+        " the ends of lines before calling setline(), or we will see them
+        " twice.
+        let l:lines_to_set = getbufvar(l:buffer, '&fileformat') is# 'dos'
+        \   ? map(copy(l:data.output), 'substitute(v:val, ''\r\+$'', '''', '''')')
+        \   : l:data.output
+
+        call setline(1, l:lines_to_set)
 
         if l:data.should_save
             if empty(&buftype)

autoload/ale/util.vim

@@ -405,7 +405,7 @@ endfunction
 " the buffer.
 function! ale#util#Writefile(buffer, lines, filename) abort
     let l:corrected_lines = getbufvar(a:buffer, '&fileformat') is# 'dos'
-    \   ? map(copy(a:lines), 'v:val . "\r"')
+    \   ? map(copy(a:lines), 'substitute(v:val, ''\r*$'', ''\r'', '''')')
     \   : a:lines
 
     call writefile(l:corrected_lines, a:filename) " no-custom-checks

test/test_writefile_function.vader

@@ -28,7 +28,26 @@ Execute(Carriage returns should be included for ale#util#Writefile):
   AssertEqual
   \ ["first\r", "second\r", "third\r", ''],
   \ readfile('.newline-test', 'b')
-  \
+
+Given(A file with extra carriage returns):
+  first
+  second
+  third
+  fourth
+
+Execute(Carriage returns should be de-depulicated):
+  call ale#test#SetFilename('.newline-test')
+
+  setlocal buftype=
+  noautocmd :w
+  noautocmd :e! ++ff=dos
+
+  call ale#util#Writefile(bufnr(''), getline(1, '$'), '.newline-test')
+
+  AssertEqual
+  \ ["first\r", "second\r", "third\r", "fourth\r", ''],
+  \ readfile('.newline-test', 'b')
+
 Given(A file with Unix line ending characters):
   first
   second