journalctl: show duplicate entries if they are from the same file (#14898)

When having a service which intentionally outputs multiple equal lines,
all these messages might be inserted with the same timestamp.

journalctl has a mechanism to avoid duplicate lines, which might be in
different journal files.

This patch allows duplicate lines, if they are from the same file.

(cherry picked from commit b6849042)
(cherry picked from commit 2867dfbf)
(cherry picked from commit d2559885)