Prevent duplicate migrations_paths

[#50099107]

If Msf::DBManager#initialize_metasploit_data_models is run multiple
times, such as during specs, ActiveRecord::Migrator.migrations_paths was
getting populated with multiple copies of the metasploit_data_models
db/migrate path, which would lead to 'DB.migrate threw an exception:
Multiple migrations have the version number 0' errors in framework.log.

Author: limhoff-r7

lib/msf/core/db_manager.rb

@@ -163,7 +163,16 @@ def initialize_metasploit_data_models
 				'db',
 				'migrate'
 		)
-		ActiveRecord::Migrator.migrations_paths << metasploit_data_model_migrations_pathname.to_s
+		metasploit_data_model_migrations_path = metasploit_data_model_migrations_pathname.to_s
+
+		# Since ActiveRecord::Migrator.migrations_paths can persist between
+		# instances of Msf::DBManager, such as in specs,
+		# metasploit_data_models_migrations_path may already be part of
+		# migrations_paths, in which case it should not be added or multiple
+		# migrations with the same version number errors will occur.
+		unless ActiveRecord::Migrator.migrations_paths.include? metasploit_data_model_migrations_path
+			ActiveRecord::Migrator.migrations_paths << metasploit_data_model_migrations_path
+		end
 	end
 
 	#