When does below error occur?

Column names in each table must be unique. Column name 'PROJECT_ISSUE_ID' in table 'ISSUE_REGISTER' is specified more than once.

Answer:

When there is already a column named PROJECT_ISSUE_ID in table ISSUE_REGISTER and we are again adding it into the same table.
Then we get above error.Because column name must be unique and we can not add same column in a same table.