php - How to check if a value already exists to avoid duplicates? -

i've got table of urls , don't want duplicate urls. how check see if given url in table using php/mysql?

if don't want have duplicates can following:

• add uniqueness constraint
• use "replace" or "insert ... on duplicate key update" syntax

if multiple users can insert data db, method suggested @jeremy ruten, can lead error: after performed check can insert similar data table.