diff --git a/AltStore/Managing Apps/AppManager.swift b/AltStore/Managing Apps/AppManager.swift
index 191857d8..7c66b4a7 100644
--- a/AltStore/Managing Apps/AppManager.swift	
+++ b/AltStore/Managing Apps/AppManager.swift	
@@ -415,10 +415,13 @@ extension AppManager
                     switch result
                     {
                     case .success(let source): fetchedSources.insert(source)
-                    case .failure(let error):
+                    case .failure(let nsError as NSError):
                         let source = managedObjectContext.object(with: source.objectID) as! Source
-                        source.error = (error as NSError).sanitizedForSerialization()
+                        let title = String(format: NSLocalizedString("Unable to Refresh “%@” Source", comment: ""), source.name)
+                        
+                        let error = nsError.withLocalizedTitle(title)
                         errors[source] = error
+                        source.error = error.sanitizedForSerialization()
                     }
                     
                     dispatchGroup.leave()
diff --git a/AltStore/Operations/FetchSourceOperation.swift b/AltStore/Operations/FetchSourceOperation.swift
index 443cd037..49611ac1 100644
--- a/AltStore/Operations/FetchSourceOperation.swift
+++ b/AltStore/Operations/FetchSourceOperation.swift
@@ -12,6 +12,41 @@ import CoreData
 import AltStoreCore
 import Roxas
 
+extension SourceError
+{
+    enum Code: Int, ALTErrorCode
+    {
+        typealias Error = SourceError
+        
+        case unsupported
+        case duplicateBundleID
+    }
+    
+    static func unsupported(_ source: Source) -> SourceError { SourceError(code: .unsupported, source: source) }
+    static func duplicateBundleID(_ bundleID: String, source: Source) -> SourceError { SourceError(code: .duplicateBundleID, source: source, duplicateBundleID: bundleID) }
+}
+
+struct SourceError: ALTLocalizedError
+{
+    var code: Code
+    var errorTitle: String?
+    var errorFailure: String?
+    
+    @Managed var source: Source
+    var duplicateBundleID: String?
+    
+    var errorFailureReason: String {
+        switch self.code
+        {
+        case .unsupported: return String(format: NSLocalizedString("The source “%@” is not supported by this version of AltStore.", comment: ""), self.$source.name)
+        case .duplicateBundleID:
+            let bundleIDFragment = self.duplicateBundleID.map { String(format: NSLocalizedString("the bundle identifier %@", comment: ""), $0) } ?? NSLocalizedString("the same bundle identifier", comment: "")
+            let failureReason = String(format: NSLocalizedString("The source “%@” contains multiple apps with %@.", comment: ""), self.$source.name, bundleIDFragment)
+            return failureReason
+        }
+    }
+}
+
 @objc(FetchSourceOperation)
 final class FetchSourceOperation: ResultOperation<Source>
 {
@@ -78,6 +113,8 @@ final class FetchSourceOperation: ResultOperation<Source>
                     let source = try decoder.decode(Source.self, from: data)
                     let identifier = source.identifier
                     
+                    try self.verify(source)
+                    
                     try childContext.save()
                     
                     self.managedObjectContext.perform {
@@ -105,3 +142,23 @@ final class FetchSourceOperation: ResultOperation<Source>
         dataTask.resume()
     }
 }
+
+private extension FetchSourceOperation
+{
+    func verify(_ source: Source) throws
+    {
+        #if !BETA
+        if let trustedSourceIDs = UserDefaults.shared.trustedSourceIDs
+        {
+            guard trustedSourceIDs.contains(source.identifier) || source.identifier == Source.altStoreIdentifier else { throw SourceError(code: .unsupported, source: source) }
+        }
+        #endif
+        
+        var bundleIDs = Set<String>()
+        for app in source.apps
+        {
+            guard !bundleIDs.contains(app.bundleIdentifier) else { throw SourceError.duplicateBundleID(app.bundleIdentifier, source: source) }
+            bundleIDs.insert(app.bundleIdentifier)
+        }
+    }
+}
diff --git a/AltStore/Sources/SourcesViewController.swift b/AltStore/Sources/SourcesViewController.swift
index 0034f53c..96ff8798 100644
--- a/AltStore/Sources/SourcesViewController.swift
+++ b/AltStore/Sources/SourcesViewController.swift
@@ -239,7 +239,14 @@ private extension SourcesViewController
                 {
                 case .success: break
                 case .failure(OperationError.cancelled): break
-                case .failure(let error): self.present(error)
+                    
+                case .failure(var error as SourceError):
+                    let title = String(format: NSLocalizedString("“%@” could not be added to AltStore.", comment: ""), error.$source.name)
+                    error.errorTitle = title
+                    self.present(error)
+                    
+                case .failure(let error as NSError):
+                    self.present(error.withLocalizedTitle(NSLocalizedString("Unable to Add Source", comment: "")))
                 }
                 
                 self.collectionView.reloadSections([Section.trusted.rawValue])
@@ -263,10 +270,6 @@ private extension SourcesViewController
                 let sourceName = source.name
                 let managedObjectContext = source.managedObjectContext
                 
-                #if !BETA
-                guard let trustedSourceIDs = UserDefaults.shared.trustedSourceIDs, trustedSourceIDs.contains(source.identifier) else { throw SourceError(code: .unsupported, source: source) }
-                #endif
-                
                 // Hide warning when adding a featured trusted source.
                 let message = isTrusted ? nil : NSLocalizedString("Make sure to only add sources that you trust.", comment: "")
                 
@@ -311,9 +314,10 @@ private extension SourcesViewController
         }
         
         let nsError = error as NSError
-        let message = nsError.userInfo[NSDebugDescriptionErrorKey] as? String ?? nsError.localizedRecoverySuggestion
+        let title = nsError.localizedTitle // OK if nil.
+        let message = [nsError.localizedDescription, nsError.localizedDebugDescription, nsError.localizedRecoverySuggestion].compactMap { $0 }.joined(separator: "\n\n")
         
-        let alertController = UIAlertController(title: error.localizedDescription, message: message, preferredStyle: .alert)
+        let alertController = UIAlertController(title: title, message: message, preferredStyle: .alert)
         alertController.addAction(.ok)
         self.present(alertController, animated: true, completion: nil)
     }