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

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 b6849042d610da90d5821a03967d648d424f7864)
(cherry picked from commit 2867dfbf70a5d761f662fe4b7c81a67e19df008b)