Check for duplicates in sqlite3 database and return error if any

Well, here am i again, now i wanted to ask this forum if anyone can help me. I will explain, i have a sqlite3 database file with multiple tables, each one with different values, what i want to do now is to check if a value already exists in any table when inserting it to the table and if it already exists returns an error or something. This is because i'm doing a program t help nurses have a database with their patient and check if a patient has already been inserted into the database, I dont post any code because i'm gathering all the information needed before programming anything to avoid spaghetti code If someone could help me it would b fantastic