Give a nice message if there are duplicate migrations instead of raising a strange insert error

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -331,15 +331,26 @@ def initialize_schema_migrations_table
       end
 
       def assume_migrated_upto_version(version)
+        version = version.to_i
         sm_table = quote_table_name(ActiveRecord::Migrator.schema_migrations_table_name)
+
         migrated = select_values("SELECT version FROM #{sm_table}").map(&:to_i)
         versions = Dir['db/migrate/[0-9]*_*.rb'].map do |filename|
           filename.split('/').last.split('_').first.to_i
         end
 
-        execute "INSERT INTO #{sm_table} (version) VALUES ('#{version}')" unless migrated.include?(version.to_i)
-        (versions - migrated).select { |v| v < version.to_i }.each do |v|
-          execute "INSERT INTO #{sm_table} (version) VALUES ('#{v}')"
+        unless migrated.include?(version)
+          execute "INSERT INTO #{sm_table} (version) VALUES ('#{version}')"
+        end
+
+        inserted = Set.new
+        (versions - migrated).each do |v|
+          if inserted.include?(v)
+            raise "Duplicate migration #{v}. Please renumber your migrations to resolve the conflict."
+          elsif v < version
+            execute "INSERT INTO #{sm_table} (version) VALUES ('#{v}')"
+            inserted << v
+          end
         end
       end