Need help with Table Relationship

Is the New Joiners table expected to have duplicate values?

I mean how can an employee join twice, once in june and other in July?

If it is acceptable as per your data model, then you would have to look for some other field to create a relationship on.

Or else, create a bridge table with all the unique values of employee between New Joiner and Headcount and create a relationship of the two tables with the bridge table.

It's always better to have information related to employee in a separate table.