how to set a column as unique indexer on Sqlite

Question

I have 3 columns (_id, column1, column2) _id column has been set as autoincrement

In database there are some duplicate records, so I want to prevent duplicate record

Answer 1

No magic, just SQL:

create table yourtablename (_id  integer primary key autoincrement, column1 text not null unique, column2 text);

_id will not be duplicate in any way because it is primary key, column1 neither because it is unique.