Prevent loading indicator from potentially being added multiple times

Not sure how this could happen, but it's caused 1 crash in the wild so w/e

Tusker/Screens/Utilities/DiffableTimelineLikeTableViewController.swift

@@ -25,6 +25,7 @@ class DiffableTimelineLikeTableViewController<Section: DiffableTimelineLikeSecti
     
     private(set) var state = State.unloaded
     private var lastLastVisibleRow: IndexPath?
+    private var currentLoadingIndicatorWorkItem: DispatchWorkItem?
     
     private(set) var dataSource: UITableViewDiffableDataSource<Section, Item>!
     
@@ -113,13 +114,24 @@ class DiffableTimelineLikeTableViewController<Section: DiffableTimelineLikeSecti
     }
     
     private func showLoadingIndicatorDelayed() -> DispatchWorkItem {
+        currentLoadingIndicatorWorkItem?.cancel()
         let workItem = DispatchWorkItem { [weak self] in
             guard let self = self else { return }
             var snapshot = self.dataSource.snapshot()
-            snapshot.appendSections([.loadingIndicator])
-            snapshot.appendItems([.loadingIndicator])
-            self.dataSource.apply(snapshot, animatingDifferences: false)
+            var changed = false
+            if !snapshot.sectionIdentifiers.contains(.loadingIndicator) {
+                snapshot.appendSections([.loadingIndicator])
+                changed = true
+            }
+            if changed || !snapshot.itemIdentifiers(inSection: .loadingIndicator).contains(.loadingIndicator) {
+                snapshot.appendItems([.loadingIndicator], toSection: .loadingIndicator)
+                changed = true
+            }
+            if changed {
+                self.dataSource.apply(snapshot, animatingDifferences: false)
+            }
         }
+        currentLoadingIndicatorWorkItem = workItem
         DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(250), execute: workItem)
         return workItem
     }