Table schema:
CREATE TABLE products (product_id VARCHAR); CREATE TABLE CHARACTERISTICS (characteristic_name VARCHAR, characteristic_id VARCHAR); CREATE TABLE product_characteristics (product_id VARCHAR, characteristic_id VARCHAR)

Question: What are characteristic names used at least twice across all products?
SQL: SELECT t3.characteristic_name FROM products AS t1 JOIN product_characteristics AS t2 ON t1.product_id = t2.product_id JOIN CHARACTERISTICS AS t3 ON t2.characteristic_id = t3.characteristic_id GROUP BY t3.characteristic_name HAVING COUNT(*) >= 2