diff --git a/addons/sale_timesheet/models/project_sale_line_employee_map.py b/addons/sale_timesheet/models/project_sale_line_employee_map.py
index 27b0afe2826..b1809db9e33 100644
--- a/addons/sale_timesheet/models/project_sale_line_employee_map.py
+++ b/addons/sale_timesheet/models/project_sale_line_employee_map.py
@@ -9,7 +9,8 @@ class ProjectProductEmployeeMap(models.Model):
     _description = 'Project Sales line, employee mapping'
 
     project_id = fields.Many2one('project.project', "Project", required=True)
-    employee_id = fields.Many2one('hr.employee', "Employee", required=True)
+    employee_id = fields.Many2one('hr.employee', "Employee", required=True, domain="[('id', 'not in', existing_employee_ids)]")
+    existing_employee_ids = fields.Many2many('hr.employee', compute="_compute_existing_employee_ids")
     sale_line_id = fields.Many2one(
         'sale.order.line', "Sales Order Item",
         compute="_compute_sale_line_id", store=True, readonly=False,
@@ -34,6 +35,15 @@ class ProjectProductEmployeeMap(models.Model):
         ('uniqueness_employee', 'UNIQUE(project_id,employee_id)', 'An employee cannot be selected more than once in the mapping. Please remove duplicate(s) and try again.'),
     ]
 
+    @api.depends('employee_id', 'project_id.sale_line_employee_ids.employee_id')
+    def _compute_existing_employee_ids(self):
+        project = self.project_id
+        if len(project) == 1:
+            self.existing_employee_ids = project.sale_line_employee_ids.employee_id
+            return
+        for map_entry in self:
+            map_entry.existing_employee_ids = map_entry.project_id.sale_line_employee_ids.employee_id
+
     @api.depends('partner_id')
     def _compute_sale_line_id(self):
         self.filtered(
diff --git a/addons/sale_timesheet/views/project_task_views.xml b/addons/sale_timesheet/views/project_task_views.xml
index 74d8a71a136..c166c78b37e 100644
--- a/addons/sale_timesheet/views/project_task_views.xml
+++ b/addons/sale_timesheet/views/project_task_views.xml
@@ -12,12 +12,13 @@
             </xpath>
             <xpath expr="//page[@name='settings']" position="after">
                 <page name="billing_employee_rate" string="Invoicing" attrs="{'invisible': ['|', ('allow_billable', '=', False), ('partner_id', '=', False)]}">
-                    <field name="sale_line_employee_ids">
+                    <field name="sale_line_employee_ids" context="{'default_sale_line_id': sale_line_id}">
                         <tree editable="bottom">
                             <field name="company_id" invisible="1"/>
                             <field name="partner_id" invisible="1"/>
                             <field name="sale_order_id" invisible="1"/>
-                            <field name="employee_id" options="{'no_create': True}" widget="many2one_avatar_user"/>
+                            <field name="employee_id" widget="many2one_avatar_user"/>
+                            <field name="existing_employee_ids" invisible="1"/>
                             <field name="sale_line_id" attrs="{'required': True}" options="{'no_create': True}" context="{'search_default_order_id': sale_order_id}"/>
                             <field name="price_unit" widget="monetary" force_save="1" options="{'currency_field': 'currency_id'}"/>
                             <field name="display_cost" widget="monetary" options="{'currency_field': 'currency_id'}"/>