node.js - Error: Can't set headers after they are sent on post request, why? -

any time added validation schema , try make post request following error message:

error: can't set headers after sent.

var playerschema   = new schema({     firstname:  { type: string, require: true, required: 'first name required' },     lastname:   { type: string, require: true, required: 'last name required' },     nickname:   { type: string, trim: true },     mentor:     { type: string, trim: true },     clubname:   { type: string, trim: true },     created_at: { type: date, default: date.now },     updated_at: { type: date, default: date.now },      email: {         type:   string,         trim:   true,         unique: true,         required: 'email address required',         validate: [validateemail, 'please fill valid email address'],         match: [/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/, 'please fill valid email address']     },     phonenumber: {        type:     string,        unique:   true,        required: 'phone number required',      } });