diff --git a/models/login_source.go b/models/login_source.go
index 8d5d08dea6a..ffae6bfaf2e 100644
--- a/models/login_source.go
+++ b/models/login_source.go
@@ -20,9 +20,9 @@ import (
 	"github.com/go-xorm/xorm"
 
 	"code.gitea.io/gitea/modules/auth/ldap"
+	"code.gitea.io/gitea/modules/auth/oauth2"
 	"code.gitea.io/gitea/modules/auth/pam"
 	"code.gitea.io/gitea/modules/log"
-	"code.gitea.io/gitea/modules/auth/oauth2"
 )
 
 // LoginType represents an login type.
@@ -31,12 +31,12 @@ type LoginType int
 // Note: new type must append to the end of list to maintain compatibility.
 const (
 	LoginNoType LoginType = iota
-	LoginPlain   // 1
-	LoginLDAP    // 2
-	LoginSMTP    // 3
-	LoginPAM     // 4
-	LoginDLDAP   // 5
-	LoginOAuth2  // 6
+	LoginPlain            // 1
+	LoginLDAP             // 2
+	LoginSMTP             // 3
+	LoginPAM              // 4
+	LoginDLDAP            // 5
+	LoginOAuth2           // 6
 )
 
 // LoginNames contains the name of LoginType values.
@@ -498,7 +498,7 @@ func LoginViaSMTP(user *User, login, password string, sourceID int64, cfg *SMTPC
 		idx := strings.Index(login, "@")
 		if idx == -1 {
 			return nil, ErrUserNotExist{0, login, 0}
-		} else if !com.IsSliceContainsStr(strings.Split(cfg.AllowedDomains, ","), login[idx + 1:]) {
+		} else if !com.IsSliceContainsStr(strings.Split(cfg.AllowedDomains, ","), login[idx+1:]) {
 			return nil, ErrUserNotExist{0, login, 0}
 		}
 	}
@@ -589,16 +589,16 @@ func LoginViaPAM(user *User, login, password string, sourceID int64, cfg *PAMCon
 
 // OAuth2Provider describes the display values of a single OAuth2 provider
 type OAuth2Provider struct {
-	Name string
+	Name        string
 	DisplayName string
-	Image string
+	Image       string
 }
 
 // OAuth2Providers contains the map of registered OAuth2 providers in Gitea (based on goth)
 // key is used to map the OAuth2Provider with the goth provider type (also in LoginSource.OAuth2Config.Provider)
 // value is used to store display data
 var OAuth2Providers = map[string]OAuth2Provider{
-	"github":   {Name: "github", DisplayName:"GitHub", Image: "/img/github.png"},
+	"github": {Name: "github", DisplayName: "GitHub", Image: "/img/github.png"},
 }
 
 // ExternalUserLogin attempts a login using external source types.
@@ -624,6 +624,16 @@ func UserSignIn(username, password string) (*User, error) {
 	var user *User
 	if strings.Contains(username, "@") {
 		user = &User{Email: strings.ToLower(strings.TrimSpace(username))}
+		// check same email
+		cnt, err := x.Count(user)
+		if err != nil {
+			return nil, err
+		}
+		if cnt > 1 {
+			return nil, ErrEmailAlreadyUsed{
+				Email: user.Email,
+			}
+		}
 	} else {
 		user = &User{LowerName: strings.ToLower(strings.TrimSpace(username))}
 	}
diff --git a/routers/user/auth.go b/routers/user/auth.go
index 5b9297d3498..fa9f50139a2 100644
--- a/routers/user/auth.go
+++ b/routers/user/auth.go
@@ -7,20 +7,20 @@ package user
 import (
 	"errors"
 	"fmt"
+	"net/http"
 	"net/url"
-
-	"github.com/go-macaron/captcha"
+	"strings"
 
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/modules/auth"
+	"code.gitea.io/gitea/modules/auth/oauth2"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
-	"net/http"
-	"code.gitea.io/gitea/modules/auth/oauth2"
+
+	"github.com/go-macaron/captcha"
 	"github.com/markbates/goth"
-	"strings"
 )
 
 const (
@@ -66,7 +66,7 @@ func AutoSignIn(ctx *context.Context) (bool, error) {
 	}
 
 	if val, _ := ctx.GetSuperSecureCookie(
-		base.EncodeMD5(u.Rands + u.Passwd), setting.CookieRememberName); val != u.Name {
+		base.EncodeMD5(u.Rands+u.Passwd), setting.CookieRememberName); val != u.Name {
 		return false, nil
 	}
 
@@ -144,6 +144,8 @@ func SignInPost(ctx *context.Context, form auth.SignInForm) {
 	if err != nil {
 		if models.IsErrUserNotExist(err) {
 			ctx.RenderWithErr(ctx.Tr("form.username_password_incorrect"), tplSignIn, &form)
+		} else if models.IsErrEmailAlreadyUsed(err) {
+			ctx.RenderWithErr(ctx.Tr("form.email_been_used"), tplSignIn, &form)
 		} else {
 			ctx.Handle(500, "UserSignIn", err)
 		}
@@ -296,7 +298,7 @@ func handleSignInFull(ctx *context.Context, u *models.User, remember bool, obeyR
 	if remember {
 		days := 86400 * setting.LogInRememberDays
 		ctx.SetCookie(setting.CookieUserName, u.Name, days, setting.AppSubURL)
-		ctx.SetSuperSecureCookie(base.EncodeMD5(u.Rands + u.Passwd),
+		ctx.SetSuperSecureCookie(base.EncodeMD5(u.Rands+u.Passwd),
 			setting.CookieRememberName, u.Name, days, setting.AppSubURL)
 	}