- prevent adding duplicate entries in +table-pagination-status+

This could  happened when concurrent fetching  messages command was
  launched from the user.

src/db.lisp

@@ -1936,10 +1936,21 @@ to  `timeline' ,  `folder'  and possibly  `account-id', older  than
 (defun first-pagination-status-id-timeline-folder (timeline folder)
   (first-status-id-timeline-folder-table timeline folder :pagination-status))
 
-(defun add-to-pagination-status (status-id folder timeline)
+(defun find-pagination-status (status-id folder timeline)
+  (fetch-single (select :*
+                  (from +table-pagination-status+)
+                  (where (:and (:= :status-id status-id)
+                               (:= :folder    folder)
+                               (:= :timeline  timeline))))))
+
+(defun add-to-pagination-status (status-id folder timeline &key (ensure-no-duplicates nil))
+  (let ((no-duplicate-p (if ensure-no-duplicates
+                            (not (find-pagination-status status-id folder timeline))
+                            t)))
+    (when no-duplicate-p
       (query (make-insert +table-pagination-status+
                           (:status-id :folder :timeline)
-                      (status-id  folder  timeline))))
+                          (status-id  folder  timeline))))))
 
 (defun remove-pagination-status (folder timeline)
   "Removes  all  the pagination  data  (i.e.  all columns  from  table

src/package.lisp

@@ -761,6 +761,7 @@
    :first-ignored-status-id-timeline-folder
    :last-pagination-status-id-timeline-folder
    :first-pagination-status-id-timeline-folder
+   :find-pagination-status
    :add-to-pagination-status
    :remove-pagination-status
    :count-status-marked-to-delete

src/program-events.lisp

@@ -860,7 +860,7 @@
   (with-accessors ((status-id status-id)
                    (timeline  timeline)
                    (folder    folder)) object
-    (db:add-to-pagination-status status-id folder timeline)))
+    (db:add-to-pagination-status status-id folder timeline :ensure-no-duplicates t)))
 
 (defclass poll-vote-event (program-event)
   ((poll-id