Count variants only once in Spree::Variant.suppliable

Variants are counted multiple times when they have stock in multiple stock
locations. Adding `distinct` fixes this behavior. Fixes solidusio#2868

core/app/models/spree/variant.rb

@@ -121,7 +121,7 @@ def self.suppliable
         Spree::StockItem.arel_table[:count_on_hand].gt(0),
         Spree::StockItem.arel_table[:backorderable].eq(true)
       ]
-      joins(:stock_items).where(arel_conditions.inject(:or))
+      joins(:stock_items).where(arel_conditions.inject(:or)).distinct
     end
 
     self.whitelisted_ransackable_associations = %w[option_values product prices default_price]

core/spec/models/spree/variant_spec.rb

@@ -783,6 +783,10 @@
       expect( subject ).not_to include(out_of_stock_variant)
     end
 
+    it "includes variants only once" do
+      expect(subject.to_a.count(in_stock_variant)).to be 1
+    end
+
     context "inventory levels globally not tracked" do
       before { Spree::Config.track_inventory_levels = false }