Need some advice for a Many to many fluent code problem - entity-framework

Question

Hi I got 2 Many to Many table linked with a joint table.
UsagerEW <-> UsagerPermissionEW <-> Permission
This is an old DB and the UsagerEW do not have a Id column but a CodeInt instead.
When I scaffold this database, EF7 create the 3 tables and relations between all. But do not skip the join table to create a "pure" many to many relationships.
So I add 2 virtual properties in each table:
In permission: public virtual ICollection UsagerEWs { get; } = new HashSet();
In UsagerEW: public virtual ICollection Permissions { get; } = new HashSet();
Then I add this fluent code to create a Many2Many relationship:
modelBuilder.Entity<PermissionUsagerEW>().HasKey(k => k.id);
modelBuilder.Entity<UsagerEW>()
.HasMany(u => u.Permissions)
.WithMany(p => p.UsagerEWs)
.UsingEntity<PermissionUsagerEW>(
p => p.HasOne(e => e.permission)
.WithMany()
.HasForeignKey(e => e.permissionId),
p => p.HasOne(p => p.usagerEWcode_intNavigation)
.WithMany()
.HasForeignKey(e => e.usagerEWcode_int)
);
That code works great. I can read UsagerEW with permission directly. The only time this code failed is when I try to add permission to a UsagerEW.
I try to do: context.usagerEW.Permissions.add(new permission)
Or I try to create the UsagerPermission joint object and update the middle table directly. In the 2 case, I got this error:
The column name 'permissionId' is specified more than once in the SET clause or column list of an INSERT.
And to SQL code generated is
SET NOCOUNT ON;
INSERT INTO [PermissionUsagerEW] ([Permissionid], [UsagerEWcode_int], [permissionId], [usagerEWcode_int])
VALUES (#p0, #p1, #p2, #p3);
SELECT [id]
FROM [PermissionUsagerEW]
WHERE ##ROWCOUNT = 1 AND [id] = scope_identity();
So something is wrong with my fluent code.
Any idea??