payment requests: block if there are duplicated validated PFMPs

app/models/asp/payment_request_state_machine.rb

@@ -69,6 +69,12 @@ class PaymentRequestStateMachine
       request.schooling.attributive_decision.attached?
     end
 
+    guard_transition(to: :ready) do |request|
+      request.pfmp.duplicates.none? do |pfmp|
+        pfmp.in_state?(:validated)
+      end
+    end
+
     guard_transition(from: :ready, to: :sent) do |request|
       request.asp_request.present?
     end

app/models/pfmp.rb

@@ -112,4 +112,10 @@ def can_be_modified?
   def payment_due?
     day_count.present?
   end
+
+  def duplicates
+    student.pfmps.excluding(self).select do |other|
+      other.start_date == start_date && other.end_date == end_date
+    end
+  end
 end

spec/models/asp/payment_request_state_machine_spec.rb

@@ -92,12 +92,36 @@
     end
 
     context "when the attributive decision has not been attached" do
-      before do
-        asp_payment_request.pfmp.schooling.attributive_decision.detach
-      end
+      before { asp_payment_request.pfmp.schooling.attributive_decision.detach }
 
       it_behaves_like "a blocked request"
     end
+
+    context "when there is another duplicated PFMP" do
+      let(:duplicate) do
+        pfmp = asp_payment_request.pfmp
+
+        create(
+          :pfmp,
+          schooling: pfmp.schooling,
+          start_date: pfmp.start_date,
+          end_date: pfmp.end_date,
+          day_count: pfmp.day_count
+        )
+      end
+
+      context "when it is validated" do
+        before { duplicate.validate! }
+
+        it_behaves_like "a blocked request"
+      end
+
+      context "when it's not validated" do
+        it "allows the transition" do
+          expect { asp_payment_request.mark_ready! }.not_to raise_error
+        end
+      end
+    end
   end
 
   describe "mark_as_sent!" do