How to auto increment a set of rows while inserting with the same value, safely?

Question

Let\'s take this table :

 id | ownerid |
----+---------+
  1 |      98 |
  1 |      99 |
  2 |     100 |
  2 |     101 |
  2 |     102 |
  3 |     103 |
  3 |