C compiler error when using statement list expression with a template

Couldn't think of a better title, code speaks for itself.
Found in https://github.com/nocturn9x/japl

Example

proc newString(data: int): int = 
  result = data

template addObject(s: int): untyped =
  discard s
  s

proc slice() =
  let obj = addObject(newString((var intval = 5; intval)))

slice()

Current Output

/home/dian/.cache/nim/a_d/@ma.nim.c: In function ‘slice__GUeAIxBAJ79bdeFn6EpeyzA’:
/home/dian/.cache/nim/a_d/@ma.nim.c:94:5: error: redeclaration of ‘intval’ with no linkage
   94 |  NI intval;
      |     ^~~~~~
/home/dian/.cache/nim/a_d/@ma.nim.c:92:5: note: previous declaration of ‘intval’ was here
   92 |  NI intval;
      |     ^~~~~~

Expected Output

Successful compilation?

Possible Solution

$ nim -v
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2020-10-18
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 82adefa9d10485478456ff75a700d982471708d2
active boot switches: -d:release -d:danger

[nocturn9x]

Glad to see I helped spotting a bug! Unfortunately my competence pretty much ends here though 😢

[demotomohiro]

Smaller repro code:

template foo(x: typed) =
  x
  x

proc bar =
  foo:
    var a = 1

bar()

It is equivalent to following code and should produces compile error like Error: redefinition of 'a'.

proc bar =
  var a = 1
  var a = 1

bar()

[demotomohiro]

The same error is generated with macros:

import std/macros

macro foo(x: typed): untyped =
  result = newStmtList()
  result.add x
  result.add x

proc bar =
  foo:
    var a = 1

bar()