diff --git a/app/controllers/gestionnaires/dossiers_controller.rb b/app/controllers/gestionnaires/dossiers_controller.rb
index af0f42b6f..685db3f81 100644
--- a/app/controllers/gestionnaires/dossiers_controller.rb
+++ b/app/controllers/gestionnaires/dossiers_controller.rb
@@ -80,8 +80,12 @@ module Gestionnaires
     end
 
     def passer_en_instruction
-      dossier.passer_en_instruction!(current_gestionnaire)
-      flash.notice = 'Dossier passé en instruction.'
+      if dossier.en_instruction?
+        flash.notice = 'Le dossier est déjà en instruction.'
+      else
+        dossier.passer_en_instruction!(current_gestionnaire)
+        flash.notice = 'Dossier passé en instruction.'
+      end
 
       render partial: 'state_button_refresh', locals: { dossier: dossier }
     end
diff --git a/spec/controllers/gestionnaires/dossiers_controller_spec.rb b/spec/controllers/gestionnaires/dossiers_controller_spec.rb
index 570f6b3c4..ca53d1210 100644
--- a/spec/controllers/gestionnaires/dossiers_controller_spec.rb
+++ b/spec/controllers/gestionnaires/dossiers_controller_spec.rb
@@ -105,16 +105,26 @@ describe Gestionnaires::DossiersController, type: :controller do
   end
 
   describe '#passer_en_instruction' do
+    let(:dossier) { create(:dossier, :en_construction, procedure: procedure) }
+
     before do
-      dossier.en_construction!
       sign_in gestionnaire
       post :passer_en_instruction, params: { procedure_id: procedure.id, dossier_id: dossier.id }, format: 'js'
-      dossier.reload
     end
 
-    it { expect(dossier.state).to eq(Dossier.states.fetch(:en_instruction)) }
-    it { expect(response.body).to include('.state-button') }
+    it { expect(dossier.reload.state).to eq(Dossier.states.fetch(:en_instruction)) }
     it { expect(gestionnaire.follow?(dossier)).to be true }
+    it { expect(response).to have_http_status(:ok) }
+    it { expect(response.body).to include('.state-button') }
+
+    context 'when the dossier has already been put en_instruction' do
+      let(:dossier) { create(:dossier, :en_instruction, procedure: procedure) }
+
+      it 'warns about the error, but doesn’t raise' do
+        expect(dossier.reload.state).to eq(Dossier.states.fetch(:en_instruction))
+        expect(response).to have_http_status(:ok)
+      end
+    end
   end
 
   describe '#repasser_en_construction' do