org-element: Fix parsing of objects of the same type in a single paragraph

* lisp/org-element.el (org-element--get-next-object-candidates): Fix
  parsing of objects of the same type in a single paragraph.
* testing/lisp/test-org-element.el: Add tests.

lisp/org-element.el

@@ -3886,7 +3886,13 @@ type, as a symbol.
 
 OBJECTS is the previous candidates alist."
   ;; Filter out any object found but not belonging to RESTRICTION.
-  (setq objects (org-remove-if-not (lambda (obj) (memq (car obj) restriction))
+  (setq objects
+	(org-remove-if-not
+	 (lambda (obj)
+	   (let ((type (car obj)))
+	     (memq (or (cdr (assq type org-element-object-successor-alist))
+		       type)
+		   restriction)))
 	 objects))
   (let (next-candidates types-to-search)
     ;; If no previous result, search every object type in RESTRICTION.

testing/lisp/test-org-element.el

@@ -1523,7 +1523,13 @@ Outside list"
    (eq 'subscript
        (org-test-with-temp-text "- _b"
 	 (progn (search-forward "_")
-		(org-element-type (org-element-context)))))))
+		(org-element-type (org-element-context))))))
+  ;; Multiple subscripts in a paragraph.
+  (should
+   (= 2
+      (org-test-with-temp-text "a_b and c_d"
+	(length
+	 (org-element-map (org-element-parse-buffer) 'subscript 'identity))))))
 
 
 ;;;; Superscript
@@ -1543,7 +1549,14 @@ Outside list"
    (eq 'superscript
        (org-test-with-temp-text "- ^b"
 	 (progn (search-forward "^")
-		(org-element-type (org-element-context)))))))
+		(org-element-type (org-element-context))))))
+  ;; Multiple superscript in a paragraph.
+  (should
+   (= 2
+      (org-test-with-temp-text "a^b and c^d"
+	(length
+	 (org-element-map
+	  (org-element-parse-buffer) 'superscript 'identity))))))
 
 
 ;;;; Table