failing to set the appveyor version should not be a fatal build error

build.cake

@@ -287,6 +287,18 @@ Task("UpdateAppVeyorBuildNumber")
     .Does(() =>
 {
     AppVeyor.UpdateBuildVersion(buildVersion);
+
+}).ReportError(exception =>
+{  
+    // When a build starts, the initial identifier is an auto-incremented value supplied by AppVeyor. 
+    // As part of the build script, this version in AppVeyor is changed to be the version obtained from
+    // GitVersion. This identifier is purely cosmetic and is used by the core team to correlate a build
+    // with the pull-request. In some circumstances, such as restarting a failed/cancelled build the
+    // identifier in AppVeyor will be already updated and default behaviour is to throw an
+    // exception/cancel the build when in fact it is safe to swallow.
+    // See https://github.com/reactiveui/ReactiveUI/issues/1262
+
+    Warning("Build with version {0} already exists.", buildVersion);
 });
 
 Task("UpdateAssemblyInfo")