/sys/kernel/debug/tracing/trace overwrites itself after a few seconds. How can I collect it without duplicates?

From ftrace documentation:

trace_pipe:

The output is the same as the "trace" file but this
file is meant to be streamed with live tracing. 
Reads from this file will block until new data is
retrieved.  Unlike the "trace" file, this file is a
consumer.  This means reading from this file causes
sequential reads to display more current data.  Once
data is read from this file, it is consumed, and
will not be read again with a sequential read. 
The "trace" file is static, and if the tracer is not
adding more data, it will display the same
information every time it is read. 
This file will not disable tracing while being read.

Or use trace-cmd(1), and specifically trace-cmd-show. 
From its man page:

-p                        (lower case ‘P’)

Instead of displaying the contents of the "trace" file, use
the "trace_pipe" file. The difference between the two is that
the "trace" file is static. That is, if tracing is stopped,
the "trace" file will show the same contents each time.

The "trace_pipe" file is a consuming read, where a read of the file
will consume the output of what was read and it will not read the
same thing a second time even if tracing is stopped. This file
als will block. 
If no data is available, trace-cmd show will stop
and wait for data to appear.

Also see Knio’s answer to How to set the buffer size
for trace_pipe in ftrace? (on Stack Overflow)
to see how you can change the size and behavior of the trace file.