diff --git a/autoload/airline/init.vim b/autoload/airline/init.vim
index 37ac856..aa93e1e 100644
--- a/autoload/airline/init.vim
+++ b/autoload/airline/init.vim
@@ -61,6 +61,10 @@ function! airline#init#bootstrap()
         \ 'modified': '+',
         \ 'space': ' ',
         \ }, 'keep')
+endfunction
+
+function! airline#init#sections()
+  let g:airline#init#initializing_sections = 1
 
   call airline#parts#define('mode', {
         \ 'function': 'airline#parts#mode',
@@ -77,9 +81,7 @@ function! airline#init#bootstrap()
   call airline#parts#define_raw('linenr', (g:airline_symbols.linenr).'%#__accent_bold#%4l%#__restore__#')
   call airline#parts#define_function('ffenc', 'airline#parts#ffenc')
   call airline#parts#define_empty(['hunks', 'branch', 'tagbar', 'syntastic', 'whitespace'])
-endfunction
 
-function! airline#init#sections()
   let spc = g:airline_symbols.space
   if !exists('g:airline_section_a')
     let g:airline_section_a = airline#section#create_left(['mode', 'paste', 'iminsert'])
@@ -105,5 +107,7 @@ function! airline#init#sections()
   if !exists('g:airline_section_warning')
     let g:airline_section_warning = airline#section#create(['syntastic', 'whitespace'])
   endif
+
+  unlet g:airline#init#initializing_sections
 endfunction
 
diff --git a/autoload/airline/parts.vim b/autoload/airline/parts.vim
index e5cbd38..0a43b97 100644
--- a/autoload/airline/parts.vim
+++ b/autoload/airline/parts.vim
@@ -7,7 +7,11 @@ let s:parts = {}
 
 function! airline#parts#define(key, config)
   let s:parts[a:key] = get(s:parts, a:key, {})
-  call extend(s:parts[a:key], a:config)
+  if exists('g:airline#init#initializing_sections')
+    call extend(s:parts[a:key], a:config, 'keep')
+  else
+    call extend(s:parts[a:key], a:config, 'force')
+  endif
 endfunction
 
 function! airline#parts#define_function(key, name)
diff --git a/t/init.vim b/t/init.vim
index 9891c2d..e6e3eb6 100644
--- a/t/init.vim
+++ b/t/init.vim
@@ -6,10 +6,11 @@ function! s:clear()
   endfor
 endfunction
 
-describe 'init'
+call airline#init#bootstrap()
+
+describe 'init sections'
   before
     call s:clear()
-    call airline#init#bootstrap()
     call airline#init#sections()
   end
 
@@ -65,3 +66,11 @@ describe 'init'
   end
 end
 
+describe 'init parts'
+  it 'should not redefine parts already defined'
+    call airline#parts#define_raw('linenr', 'bar')
+    call airline#init#sections()
+    Expect g:airline_section_z =~ 'bar'
+  end
+end
+