remove redundant highlight group.

autoload/airline/builder.vim

@@ -19,7 +19,11 @@ function! s:prototype.add_section(group, contents)
     endif
   endif
 
-  let self._line .= '%#'.a:group.'#'.a:contents
+  if self._curgroup != a:group
+    let self._line .= '%#'.a:group.'#'
+  endif
+
+  let self._line .= a:contents
   let self._curgroup = a:group
 endfunction
 

t/builder.vim

@@ -30,6 +30,13 @@ describe 'active builder'
     let stl = s:builder.build()
     Expect stl =~ '%#Normal#hello%#Normal_to_NonText#<%#NonText#world'
   end
+
+  it 'should not repeat the same highlight group'
+    call s:builder.add_section('Normal', 'hello')
+    call s:builder.add_section('Normal', 'hello')
+    let stl = s:builder.build()
+    Expect stl == '%#Normal#hello>hello'
+  end
 end
 
 describe 'inactive builder'