title: How to prevent field value repetition on already assigned date in Odoo?
body: <p>I am working with Odoo 10.</p>
  
  <p>I have a <code>one2many</code> field with two columns in the <code>hr.employee</code> model. If the field "Bonus" (<code>many2one</code> field) is assigned to a particular date, it should not be saved or repeated once again on the same date.</p>
  
  <p>How to achieve this?</p>
  
  <p><a href="https://i.stack.imgur.com/ciO8N.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ciO8N.png" alt="Current screen"></a></p>