Fix synchronous TLS-in-TLS streams (#840)

* Fix synchronous TLS-in-TLS

* changelog

Author: karpetrosyan

CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ## Unreleased
 
 - Fix pool timeout to account for the total time spent retrying. (#823)
+- Fix synchronous TLS-in-TLS streams. (#840)
 
 ## 1.0.0 (October 6th, 2023)
 

httpcore/_backends/sync.py

@@ -145,12 +145,6 @@ def start_tls(
         server_hostname: typing.Optional[str] = None,
         timeout: typing.Optional[float] = None,
     ) -> NetworkStream:
-        if isinstance(self._sock, ssl.SSLSocket):  # pragma: no cover
-            raise RuntimeError(
-                "Attempted to add a TLS layer on top of the existing "
-                "TLS stream, which is not supported by httpcore package"
-            )
-
         exc_map: ExceptionMapping = {
             socket.timeout: ConnectTimeout,
             OSError: ConnectError,