Return only unique accounts from credential stores (#1369)

If may be possible for multiple credentials for the same user account to
exist in any credential store. To avoid issues with redundant account
selection prompts we should ensure duplicate accounts are removed from
the results of `ICredentialStore.GetAccounts`.

Call `.Distinct()` or use a `HashSet` where appropriate on all
implementation of `ICredentialStore.GetAccounts`.

If there are multiple credentials for the same account, we'd only ever
be able to retrieve the first enumerated credential regardless because
our matching logic is to return to first credential that matches.

Fixes #1368

Author: mjcheetham

src/shared/Core/Interop/Linux/SecretServiceCollection.cs

@@ -39,7 +39,7 @@ public SecretServiceCollection(string @namespace)
 
         public IList<string> GetAccounts(string service)
         {
-            return Enumerate(service, null).Select(x => x.Account).ToList();
+            return Enumerate(service, null).Select(x => x.Account).Distinct().ToList();
         }
 
         public ICredential Get(string service, string account)

src/shared/Core/Interop/MacOS/MacOSKeychain.cs

@@ -66,15 +66,15 @@ public IList<string> GetAccounts(string service)
                         if (typeId == CFArrayGetTypeID())
                         {
                             int len = (int)CFArrayGetCount(resultPtr);
-                            var accounts = new List<string>(len);
+                            var accounts = new HashSet<string>(len);
                             for (int i = 0; i < len; i++)
                             {
                                 IntPtr dict = CFArrayGetValueAtIndex(resultPtr, i);
                                 string account = GetStringAttribute(dict, kSecAttrAccount);
                                 accounts.Add(account);
                             }
 
-                            return accounts;
+                            return accounts.ToList();
                         }
 
                         throw new InteropException($"Unknown keychain search result type CFTypeID: {typeId}.", -1);

src/shared/Core/Interop/Windows/WindowsCredentialManager.cs

@@ -26,7 +26,7 @@ public WindowsCredentialManager(string @namespace = null)
 
         public IList<string> GetAccounts(string service)
         {
-            return Enumerate(service, null).Select(x => x.UserName).ToList();
+            return Enumerate(service, null).Select(x => x.UserName).Distinct().ToList();
         }
 
         public ICredential Get(string service, string account)

src/shared/Core/PlaintextCredentialStore.cs

@@ -25,7 +25,7 @@ public PlaintextCredentialStore(IFileSystem fileSystem, string storeRoot, string
 
         public IList<string> GetAccounts(string service)
         {
-            return Enumerate(service, null).Select(x => x.Account).ToList();
+            return Enumerate(service, null).Select(x => x.Account).Distinct().ToList();
         }
 
         public ICredential Get(string service, string account)

src/shared/TestInfrastructure/Objects/TestCredentialStore.cs

@@ -16,7 +16,7 @@ public TestCredentialStore()
 
         public IList<string> GetAccounts(string service)
         {
-            return Query(service, null).Select(x => x.Account).ToList();
+            return Query(service, null).Select(x => x.Account).Distinct().ToList();
         }
 
         ICredential ICredentialStore.Get(string service, string account)