Migrations will raise an exception if there are multiple column definitions (same name).

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Migrations raise when duplicate column definition.
+
+    Fixes #33024.
+
+    *Federico Martinez*
+    
 *   Bump minimum SQLite version to 3.8
 
     *Yasuo Honda*

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

@@ -356,8 +356,12 @@ module ActiveRecord
         type = type.to_sym if type
         options = options.dup
 
-        if @columns_hash[name] && @columns_hash[name].primary_key?
+        if @columns_hash[name]
+          if @columns_hash[name].primary_key?
             raise ArgumentError, "you can't redefine the primary key column '#{name}'. To define a custom primary key, pass { id: false } to create_table."
+          else
+            raise ArgumentError, "you can't define an already defined column '#{name}'."
+          end
         end
 
         index_options = options.delete(:index)

activerecord/test/cases/migration/change_schema_test.rb

@@ -196,6 +196,17 @@ module ActiveRecord
         assert_equal "you can't redefine the primary key column 'testing_id'. To define a custom primary key, pass { id: false } to create_table.", error.message
       end
 
+      def test_create_table_raises_when_defining_existing_column
+        error = assert_raise(ArgumentError) do
+          connection.create_table :testings do |t|
+            t.column :testing_column, :string
+            t.column :testing_column, :integer
+          end
+        end
+
+        assert_equal "you can't define an already defined column 'testing_column'.", error.message
+      end
+
       def test_create_table_with_timestamps_should_create_datetime_columns
         connection.create_table table_name do |t|
           t.timestamps