diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 2027c2e..b261d58 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -9673,9 +9673,13 @@
 			deleted if there are no more references to it.
 								*E127* *E122*
 			When a function by this name already exists and [!] is
-			not used an error message is given.  When [!] is used,
-			an existing function is silently replaced.  Unless it
-			is currently being executed, that is an error.
+			not used an error message is given.  There is one
+			exception: When sourcing a script again, a function
+			that was previously defined in that script will be
+			silently replaced.
+			When [!] is used, an existing function is silently
+			replaced.  Unless it is currently being executed, that
+			is an error.
 			NOTE: Use ! wisely.  If used without care it can cause
 			an existing function to be replaced unexpectedly,
 			which is hard to debug.
diff --git a/src/buffer.c b/src/buffer.c
index cbccc70..8e892da 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5519,6 +5519,7 @@
 #ifdef FEAT_EVAL
 		save_current_sctx = current_sctx;
 		current_sctx.sc_sid = SID_MODELINE;
+		current_sctx.sc_seq = 0;
 		current_sctx.sc_lnum = 0;
 #endif
 		retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index 3a4bce8..d6b6f5b 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -4344,6 +4344,7 @@
 #ifdef FEAT_EVAL
     sctx_T		    save_current_sctx;
     static scid_T	    last_current_SID = 0;
+    static int		    last_current_SID_seq = 0;
     funccal_entry_T	    funccalp_entry;
     int			    save_debug_break_level = debug_break_level;
     scriptitem_T	    *si = NULL;
@@ -4508,11 +4509,11 @@
      * Also starts profiling timer for nested script. */
     save_funccal(&funccalp_entry);
 
-    /*
-     * Check if this script was sourced before to finds its SID.
-     * If it's new, generate a new SID.
-     */
+    // Check if this script was sourced before to finds its SID.
+    // If it's new, generate a new SID.
+    // Always use a new sequence number.
     save_current_sctx = current_sctx;
+    current_sctx.sc_seq = ++last_current_SID_seq;
     current_sctx.sc_lnum = 0;
 # ifdef UNIX
     stat_ok = (mch_stat((char *)fname_exp, &st) >= 0);
diff --git a/src/globals.h b/src/globals.h
index 511479d..131c13b 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -326,7 +326,7 @@
 EXTERN int	garbage_collect_at_exit INIT(= FALSE);
 
 // Script CTX being sourced or was sourced to define the current function.
-EXTERN sctx_T	current_sctx INIT(= {0 COMMA 0});
+EXTERN sctx_T	current_sctx INIT(= {0 COMMA 0 COMMA 0});
 #endif
 
 EXTERN int	did_source_packages INIT(= FALSE);
diff --git a/src/main.c b/src/main.c
index d4ff370..d24eafa 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2953,6 +2953,7 @@
     sourcing_name = (char_u *)"command line";
 #ifdef FEAT_EVAL
     current_sctx.sc_sid = SID_CARG;
+    current_sctx.sc_seq = 0;
 #endif
     for (i = 0; i < parmp->n_commands; ++i)
     {
@@ -3183,6 +3184,7 @@
 #ifdef FEAT_EVAL
 	save_current_sctx = current_sctx;
 	current_sctx.sc_sid = SID_ENV;
+	current_sctx.sc_seq = 0;
 	current_sctx.sc_lnum = 0;
 #endif
 	do_cmdline_cmd(initstr);
diff --git a/src/option.c b/src/option.c
index 3c54ab6..a4a9c97 100644
--- a/src/option.c
+++ b/src/option.c
@@ -415,7 +415,7 @@
     char_u	*def_val[2];	// default values for variable (vi and vim)
 #ifdef FEAT_EVAL
     sctx_T	script_ctx;	// script context where the option was last set
-# define SCTX_INIT , {0, 0}
+# define SCTX_INIT , {0, 0, 0}
 #else
 # define SCTX_INIT
 #endif
@@ -5959,6 +5959,7 @@
 	    else
 	    {
 		script_ctx.sc_sid = set_sid;
+		script_ctx.sc_seq = 0;
 		script_ctx.sc_lnum = 0;
 	    }
 	    set_option_sctx_idx(idx, opt_flags, script_ctx);
diff --git a/src/structs.h b/src/structs.h
index a0c06b5..cd9b5a5 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -84,6 +84,7 @@
  */
 typedef struct {
     scid_T	sc_sid;		// script ID
+    int		sc_seq;		// sourcing sequence number
     linenr_T	sc_lnum;	// line number
 } sctx_T;
 
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index ac8a27f..d942d07 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1138,3 +1138,30 @@
   call delete('Xfuncrange2')
   bwipe!
 endfunc
+
+func Test_func_exists_on_reload()
+  call writefile(['func ExistingFunction()', 'echo "yes"', 'endfunc'], 'Xfuncexists')
+  call assert_equal(0, exists('*ExistingFunction'))
+  source Xfuncexists
+  call assert_equal(1, exists('*ExistingFunction'))
+  " Redefining a function when reloading a script is OK.
+  source Xfuncexists
+  call assert_equal(1, exists('*ExistingFunction'))
+
+  " But redefining in another script is not OK.
+  call writefile(['func ExistingFunction()', 'echo "yes"', 'endfunc'], 'Xfuncexists2')
+  call assert_fails('source Xfuncexists2', 'E122:')
+
+  delfunc ExistingFunction
+  call assert_equal(0, exists('*ExistingFunction'))
+  call writefile([
+	\ 'func ExistingFunction()', 'echo "yes"', 'endfunc',
+	\ 'func ExistingFunction()', 'echo "no"', 'endfunc',
+	\ ], 'Xfuncexists')
+  call assert_fails('source Xfuncexists', 'E122:')
+  call assert_equal(1, exists('*ExistingFunction'))
+
+  call delete('Xfuncexists2')
+  call delete('Xfuncexists')
+  delfunc ExistingFunction
+endfunc
diff --git a/src/userfunc.c b/src/userfunc.c
index f79e7f1..8bfebec 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2330,14 +2330,19 @@
 	fp = find_func(name);
 	if (fp != NULL)
 	{
-	    if (!eap->forceit)
+	    // Function can be replaced with "function!" and when sourcing the
+	    // same script again, but only once.
+	    if (!eap->forceit
+			&& (fp->uf_script_ctx.sc_sid != current_sctx.sc_sid
+			    || fp->uf_script_ctx.sc_seq == current_sctx.sc_seq))
 	    {
 		emsg_funcname(e_funcexts, name);
 		goto erret;
 	    }
 	    if (fp->uf_calls > 0)
 	    {
-		emsg_funcname(N_("E127: Cannot redefine function %s: It is in use"),
+		emsg_funcname(
+			N_("E127: Cannot redefine function %s: It is in use"),
 									name);
 		goto erret;
 	    }
diff --git a/src/version.c b/src/version.c
index b611fe7..3d72854 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    515,
+/**/
     514,
 /**/
     513,