How to remove unique index from Dotnet EF?

I have a model:

User

public class User {
 public int Id { get; set; }
 public string UserName {get; set;}
 ...
 public int? ManagerId {get; set;}
 public User Manager {get; set;}
 public int? SupervisorId {get; set;}
 public User Supervisor {get; set;}
..
}

I have checked my MySQL Db, and found that ManagerId is unique. I didn't set it as unique. Now i want to remove that using dotnet ef migrations. How to do that ?

I use donet core 2.1 at the moment.

I have test with remove the ManagerID and Manager and do dotnet ef migrations add UpdateUser

checked and ManagerID is removed. then I tried to add again new name AssignedManagerID and it created new one with unique is true.

migrationBuilder.CreateIndex(
                name: "IX_Users_AssignedManagerId",
                table: "Users",
                column: "AssignedManagerId",
                unique: true);

my Datacontext: public DbSet Users {get; set;}

I have no Fluent at all on OnModelCreating() for User