ERR_HTTP_HEADERS_SENT error when visiting parked pages ending with forward slash

## To Reproduce
As discussed in https://discord.com/channels/517772094482677790/701815369005924374/912684621043486770

I created a piece page type and piece – article-page and article. It’s a blog with articles. It’s been parked at the url /spravy. When I create a new article and select publish and view I get redirected to the article http://localhost:3000/spravy/new-article which is fine. Also when you visit the parked page without the forward slash at the end it works fine. But when you visit parked page and leave a forward slash at the end http://localhost:3000/spravy/ the server crashes and I get ERR_HTTP_HEADERS_SENT.

Step by step instructions to reproduce the behavior:
I'm sharing a simple test project here to that reproduces the issue.
https://github.com/Miselrkba/apos-http-err
Visit http://localhost:3000/spravy/ and the server will crash.

**Screenshots**
*If applicable, add screenshots to help explain your problem.*

Start with the linked reproduction project, then visit http://localhost:3000/spravy/ and compare it with the working URL without the trailing slash. Trace the request handling for the parked page and determine why the server sends headers more than once; done means the trailing-slash URL no longer crashes and the parked page responds normally.

Written by the indexing model from the issue text.