Seen here:
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/linux_chromium_rel_ng/67912

It caused the failure of WebglConformance_conformance_glsl_variables_gl_fragcoord :

https://chromium-swarm.appspot.com/task?id=3ccf4297f3b85c10&refresh=10&show_raw=1

[395/461] gpu_tests.webgl_conformance_integration_test.WebGLConformanceIntegrationTest.WebglConformance_conformance_glsl_samplers_glsl_function_texture2dprojlod passed 0.2895s
[1:1:0411/134358.454988:FATAL:observer_list.h(243)] Check failed: false. Observers can only be added once!
#0 0x7fbbc86bda7c base::debug::StackTrace::StackTrace()
#1 0x7fbbc86de55b logging::LogMessage::~LogMessage()
#2 0x7fbbc64ab929 base::ObserverList<>::AddObserver()
#3 0x7fbbc81532d6 blink::scheduler::TaskQueueManagerImpl::AddTaskTimeObserver()
#4 0x7fbbcc3e4d43 blink::LongTaskDetector::RegisterObserver()
#5 0x7fbbcc3e3528 blink::InteractiveDetector::SetNavigationStartTime()
#6 0x7fbbcc3bfdd3 blink::DocumentLoader::DidCommitNavigation()
#7 0x7fbbcc3bec0b blink::DocumentLoader::InstallNewDocument()
#8 0x7fbbcc3be81f blink::DocumentLoader::CommitNavigation()
#9 0x7fbbcc3bd266 blink::DocumentLoader::CommitData()
#10 0x7fbbcc3bf0e1 blink::DocumentLoader::ProcessData()
#11 0x7fbbcc3bf05d blink::DocumentLoader::DataReceived()
#12 0x7fbbc80ebd92 blink::Resource::AppendData()
#13 0x7fbbc80e5655 blink::RawResource::AppendData()
#14 0x7fbbc8117eab blink::ResourceLoader::DidReceiveData()
#15 0x7fbbccaa7be4 content::WebURLLoaderImpl::Context::OnReceivedData()
#16 0x7fbbccaa87b7 content::WebURLLoaderImpl::RequestPeerImpl::OnReceivedData()
#17 0x7fbbcc9ccb49 content::URLResponseBodyConsumer::OnReadable()
#18 0x7fbbcc9cc258 content::URLLoaderClientImpl::OnStartLoadingResponseBody()
#19 0x7fbbc62632cb network::mojom::URLLoaderClientStubDispatch::Accept()
#20 0x7fbbc8d59e02 mojo::InterfaceEndpointClient::HandleValidatedMessage()
#21 0x7fbbc8d599a6 mojo::FilterChain::Accept()
#22 0x7fbbc8d5b2b2 mojo::InterfaceEndpointClient::HandleIncomingMessage()
#23 0x7fbbc8d6288d mojo::internal::MultiplexRouter::ProcessIncomingMessage()
#24 0x7fbbc8d61c50 mojo::internal::MultiplexRouter::Accept()
#25 0x7fbbc8d599a6 mojo::FilterChain::Accept()
#26 0x7fbbc8d572bb mojo::Connector::ReadSingleMessage()
#27 0x7fbbc8d57d44 mojo::Connector::ReadAllAvailableMessages()
#28 0x7fbbc8d57ba6 mojo::Connector::OnHandleReadyInternal()
#29 0x7fbbc68a59c4 drive::(anonymous namespace)::EntryActionCallbackAdapter()
#30 0x7fbbc8d51465 mojo::SimpleWatcher::OnHandleReady()
#31 0x7fbbc8d519ee _ZN4base8internal7InvokerINS0_9BindStateIMN4mojo13SimpleWatcherEFvijRKNS3_18HandleSignalsStateEEJNS_7WeakPtrIS4_EEijS5_EEEFvvEE7RunImplIRKS9_RKNSt3__15tupleIJSB_ijS5_EEEJLm0ELm1ELm2ELm3EEEEvOT_OT0_NSI_16integer_sequenceImJXspT1_EEEE
#32 0x7fbbc86bf140 base::debug::TaskAnnotator::RunTask()
#33 0x7fbbc8156656 blink::scheduler::internal::ThreadControllerImpl::DoWork()
#34 0x7fbbc649ce68 _ZN4base8internal7InvokerINS0_9BindStateIMN7network17ResourceScheduler28ScheduledResourceRequestImplEFvNS3_12_GLOBAL__N_19StartModeEEJNS_7WeakPtrIS5_EES7_EEEFvvEE7RunOnceEPNS0_13BindStateBaseE
#35 0x7fbbc86bf140 base::debug::TaskAnnotator::RunTask()
#36 0x7fbbc86e6fc6 base::internal::IncomingTaskQueue::RunTask()
#37 0x7fbbc86e5457 base::MessageLoop::RunTask()
#38 0x7fbbc86e586a base::MessageLoop::DeferOrRunPendingTask()
#39 0x7fbbc86e5afe base::MessageLoop::DoWork()
#40 0x7fbbc86e9626 base::MessagePumpDefault::Run()
#41 0x7fbbc86e4ccc base::MessageLoop::Run()
#42 0x7fbbc8715da6 base::RunLoop::Run()
#43 0x7fbbcd0e9a21 content::RendererMain()
#44 0x7fbbc8301108 content::RunZygote()
#45 0x7fbbc83018ea content::RunNamedProcessTypeMain()
#46 0x7fbbc83029c2 content::ContentMainRunnerImpl::Run()
#47 0x7fbbc830fde9 service_manager::Main()
#48 0x7fbbc8300d94 content::ContentMain()
#49 0x7fbbc5e461b3 ChromeMain
#50 0x7fbbbd51ff45 __libc_start_main
#51 0x7fbbc5e4602a _start


I don't know how often this is happening but it seems like a potentially serious logic flaw. Marking P1 at least until this is triaged. Linking to two bugs which touched this code recently.