diff --git a/src/getchar.c b/src/getchar.c
index 4af1769..54222ec 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -3156,6 +3156,7 @@
 	int	save_m_noremap;
 	int	save_m_silent;
 	char_u	*save_m_keys;
+	char_u	*save_alt_m_keys;
 #else
 # define save_m_noremap mp->m_noremap
 # define save_m_silent mp->m_silent
@@ -3204,6 +3205,7 @@
 	save_m_noremap = mp->m_noremap;
 	save_m_silent = mp->m_silent;
 	save_m_keys = NULL;  // only saved when needed
+	save_alt_m_keys = NULL;  // only saved when needed
 
 	/*
 	 * Handle ":map <expr>": evaluate the {rhs} as an expression.  Also
@@ -3221,6 +3223,8 @@
 	    may_garbage_collect = FALSE;
 
 	    save_m_keys = vim_strsave(mp->m_keys);
+	    save_alt_m_keys = mp->m_alt != NULL
+				    ? vim_strsave(mp->m_alt->m_keys) : NULL;
 	    map_str = eval_map_expr(mp, NUL);
 
 	    // The mapping may do anything, but we expect it to take care of
@@ -3278,15 +3282,20 @@
 		noremap = save_m_noremap;
 	    else if (
 #ifdef FEAT_EVAL
-		STRNCMP(map_str, save_m_keys != NULL ? save_m_keys : mp->m_keys,
-								(size_t)keylen)
-#else
-		STRNCMP(map_str, mp->m_keys, (size_t)keylen)
+		save_m_expr ?
+		(save_m_keys != NULL
+			&& STRNCMP(map_str, save_m_keys, (size_t)keylen) == 0)
+		|| (save_alt_m_keys != NULL
+			&& STRNCMP(map_str, save_alt_m_keys,
+					    STRLEN(save_alt_m_keys)) == 0) :
 #endif
-		   != 0)
-		noremap = REMAP_YES;
-	    else
+		STRNCMP(map_str, mp->m_keys, (size_t)keylen) == 0
+		|| (mp->m_alt != NULL
+			&& STRNCMP(map_str, mp->m_alt->m_keys,
+					    STRLEN(mp->m_alt->m_keys)) == 0))
 		noremap = REMAP_SKIP;
+	    else
+		noremap = REMAP_YES;
 	    i = ins_typebuf(map_str, noremap,
 					 0, TRUE, cmd_silent || save_m_silent);
 #ifdef FEAT_EVAL
@@ -3296,6 +3305,7 @@
 	}
 #ifdef FEAT_EVAL
 	vim_free(save_m_keys);
+	vim_free(save_alt_m_keys);
 #endif
 	*keylenp = keylen;
 	if (i == FAIL)
diff --git a/src/map.c b/src/map.c
index c416c0a..91ab6e3 100644
--- a/src/map.c
+++ b/src/map.c
@@ -85,6 +85,8 @@
 
     mp = *mpp;
     vim_free(mp->m_keys);
+    if (mp->m_alt != NULL)
+	mp->m_alt->m_alt = NULL;
     vim_free(mp->m_str);
     vim_free(mp->m_orig_str);
     *mpp = mp->m_next;
@@ -213,7 +215,7 @@
     --map_locked;
 }
 
-    static int
+    static mapblock_T *
 map_add(
 	mapblock_T  **map_table,
 	mapblock_T  **abbr_table,
@@ -236,7 +238,7 @@
     mapblock_T	*mp = ALLOC_CLEAR_ONE(mapblock_T);
 
     if (mp == NULL)
-	return FAIL;
+	return NULL;
 
     // If CTRL-C has been mapped, don't always use it for Interrupting.
     if (*keys == Ctrl_C)
@@ -256,7 +258,7 @@
 	vim_free(mp->m_str);
 	vim_free(mp->m_orig_str);
 	vim_free(mp);
-	return FAIL;
+	return NULL;
     }
     mp->m_keylen = (int)STRLEN(mp->m_keys);
     mp->m_noremap = noremap;
@@ -292,7 +294,7 @@
 	mp->m_next = map_table[n];
 	map_table[n] = mp;
     }
-    return OK;
+    return mp;
 }
 
 /*
@@ -444,6 +446,7 @@
 {
     char_u	*keys;
     mapblock_T	*mp, **mpp;
+    mapblock_T	*mp_result[2] = {NULL, NULL};
     char_u	*rhs;
     char_u	*p;
     int		n;
@@ -844,6 +847,8 @@
 					retval = 4;		// no mem
 					goto theend;
 				    }
+				    if (mp->m_alt != NULL)
+					mp->m_alt = mp->m_alt->m_alt = NULL;
 				    vim_free(mp->m_str);
 				    mp->m_str = newstr;
 				    vim_free(mp->m_orig_str);
@@ -858,6 +863,7 @@
 				    mp->m_script_ctx = current_sctx;
 				    mp->m_script_ctx.sc_lnum += SOURCING_LNUM;
 #endif
+				    mp_result[keyround - 1] = mp;
 				    did_it = TRUE;
 				}
 			    }
@@ -921,18 +927,25 @@
 	    continue;	// have added the new entry already
 
 	// Get here when adding a new entry to the maphash[] list or abbrlist.
-	if (map_add(map_table, abbr_table, keys, rhs, orig_rhs,
-		    noremap, nowait, silent, mode, abbrev,
+	mp_result[keyround - 1] = map_add(map_table, abbr_table, keys,
+		    rhs, orig_rhs, noremap, nowait, silent, mode, abbrev,
 #ifdef FEAT_EVAL
 		    expr, /* sid */ 0, /* scriptversion */ 0, /* lnum */ 0,
 #endif
-		    keyround1_simplified) == FAIL)
+		    keyround1_simplified);
+	if (mp_result[keyround - 1] == NULL)
 	{
 	    retval = 4;	    // no mem
 	    goto theend;
 	}
     }
 
+    if (mp_result[0] != NULL && mp_result[1] != NULL)
+    {
+	mp_result[0]->m_alt = mp_result[1];
+	mp_result[1]->m_alt = mp_result[0];
+    }
+
 theend:
     vim_free(keys_buf);
     vim_free(alt_keys_buf);
@@ -2710,6 +2723,7 @@
     int		nowait;
     char_u	*arg;
     int		dict_only;
+    mapblock_T	*mp_result[2] = {NULL, NULL};
 
     // If first arg is a dict, then that's the only arg permitted.
     dict_only = argvars[0].v_type == VAR_DICT;
@@ -2806,12 +2820,20 @@
     do_map(MAPTYPE_UNMAP, arg, mode, is_abbr);
     vim_free(arg);
 
-    (void)map_add(map_table, abbr_table, lhsraw, rhs, orig_rhs, noremap,
-	    nowait, silent, mode, is_abbr, expr, sid, scriptversion, lnum, 0);
+    mp_result[0] = map_add(map_table, abbr_table, lhsraw, rhs, orig_rhs,
+			    noremap, nowait, silent, mode, is_abbr, expr, sid,
+			    scriptversion, lnum, 0);
     if (lhsrawalt != NULL)
-	(void)map_add(map_table, abbr_table, lhsrawalt, rhs, orig_rhs, noremap,
-		nowait, silent, mode, is_abbr, expr, sid, scriptversion,
-								      lnum, 1);
+	mp_result[1] = map_add(map_table, abbr_table, lhsrawalt, rhs, orig_rhs,
+			    noremap, nowait, silent, mode, is_abbr, expr, sid,
+			    scriptversion, lnum, 1);
+
+    if (mp_result[0] != NULL && mp_result[1] != NULL)
+    {
+	mp_result[0]->m_alt = mp_result[1];
+	mp_result[1]->m_alt = mp_result[0];
+    }
+
     vim_free(arg_buf);
 }
 #endif
diff --git a/src/structs.h b/src/structs.h
index 32c35b7..fe4704a 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1309,6 +1309,9 @@
 struct mapblock
 {
     mapblock_T	*m_next;	// next mapblock in list
+    mapblock_T	*m_alt;		// pointer to mapblock of the same mapping
+				// with an alternative form of m_keys, or NULL
+				// if there is no such mapblock
     char_u	*m_keys;	// mapped from, lhs
     char_u	*m_str;		// mapped to, rhs
     char_u	*m_orig_str;	// rhs as entered by the user
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index 1175310..654a673 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -1767,6 +1767,49 @@
   unmap <C-I>
 endfunc
 
+" Test that the first byte of rhs is not remapped if rhs starts with lhs.
+func Test_map_rhs_starts_with_lhs()
+  new
+  func MapExpr()
+    return "\<C-R>\<C-P>"
+  endfunc
+
+  for expr in [v:false, v:true]
+    if expr
+      imap <buffer><expr> <C-R> MapExpr()
+    else
+      imap <buffer> <C-R> <C-R><C-P>
+    endif
+
+    for restore in [v:false, v:true]
+      if restore
+        let saved = maparg('<C-R>', 'i', v:false, v:true)
+        iunmap <buffer> <C-R>
+        call mapset(saved)
+      endif
+
+      let @a = 'foo'
+      call feedkeys("S\<C-R>a", 'tx')
+      call assert_equal('foo', getline('.'))
+
+      let @a = 'bar'
+      call feedkeys("S\<*C-R>a", 'tx')
+      call assert_equal('bar', getline('.'))
+    endfor
+  endfor
+
+  " When two mappings are used for <C-I> and <Tab>, remapping should work.
+  imap <buffer> <C-I> <Tab>bar
+  imap <buffer> <Tab> foo
+  call feedkeys("S\<Tab>", 'xt')
+  call assert_equal('foo', getline('.'))
+  call feedkeys("S\<*C-I>", 'xt')
+  call assert_equal('foobar', getline('.'))
+
+  delfunc MapExpr
+  bwipe!
+endfunc
+
 func Test_expr_map_escape_special()
   nnoremap … <Cmd>let g:got_ellipsis += 1<CR>
   func Func()
diff --git a/src/version.c b/src/version.c
index 8afbd01..1536859 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    642,
+/**/
     641,
 /**/
     640,