+RTS -xc stack trace sometimes reported twice

Not sure if really a bug but just wanted to show. If this is a bug I'm hoping to fix this myself but I may need some guidance.

Stack trace reported by +RTS -xc is sometimes printed twice. This program reports it only once, as expected:

f :: Int -> Int
f = error "hello"

main = print (f 20)

➜  ghc_patch  ./error +RTS -xc                                                              
*** Exception (reporting due to +RTS -xc): (THUNK_1_0), stack trace: 
  Main.f,
  called from Main.CAF
  --> evaluated by: Main.main,
  called from Main.CAF
error: hello

But if I change it to this:

f :: Int -> Int
f x =
  let x = x + 20
  in x

main = print (f 20)

Stack trace is reported twice:

➜  ghc_patch  ./error +RTS -xc                                                              
*** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: 
  Main.f.x,
  called from Main.f,
  called from Main.main,
  called from Main.CAF
*** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: 
  Main.f.x,
  called from Main.f,
  called from Main.main,
  called from Main.CAF
error: <<loop>>

Can anyone confirm that this is really a bug?

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture

evaluated by: Main.main,\r\n called from Main.CAF\r\nerror: hello\r\n}}}\r\n\r\nBut if I change it to this:\r\n\r\n{{{#!haskell\r\nf :: Int -> Int\r\nf x =\r\n let x = x + 20\r\n in x\r\n\r\nmain = print (f 20)\r\n}}}\r\n\r\nStack trace is reported twice:\r\n\r\n{{{\r\n➜ ghc_patch ./error +RTS -xc \r\n*** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: \r\n Main.f.x,\r\n called from Main.f,\r\n called from Main.main,\r\n called from Main.CAF\r\n*** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: \r\n Main.f.x,\r\n called from Main.f,\r\n called from Main.main,\r\n called from Main.CAF\r\nerror: <>\r\n}}}\r\n\r\nCan anyone confirm that this is really a bug?","type_of_failure":"OtherFailure","blocking":[]} -->