pg_dump bug?

I wonder if anyone has experienced the problem I have with bloated pg_dump
files. This problem exists in both standard pg_dumps and --insert-columns
type pg_dumps. I am using postgresql 9.5 on Windows 7.

Each table appears to have a 'copy table_name .... from stdin..' in
multiple places (hundreds in some cases). The first copy statement appears
genuine, but the second and subsequent statements appear to be in the
middle of another copy clause between records. The outer copy clause is
sometimes from the same table, and sometimes from a different table.
Furthermore, from the existence of the number of backslashes \t, \\t, \\\\t
which are variable, it would appear as though nesting can be several levels
deep.

I'm not copying the actual data because apart from anything else, it's a
giant text file. But this is roughly what it looks like:

------------------------------------
copy bills from ....
Record 1 Column 1 \t Record 1 column 2 \t ....
Record 2 Column 2 \t Record 2 column 2 \t ...
......
copy adverts from .... < this is a nested copy statement unrelated to
bills, although occasionally it's another bills copy statement>

Record 88 Column 1 \t Record 88 Column 2 \t ...

-------------------------------------

So in the middle of one copy statement - in between the records being
copied, another one mysteriously appears and the records are duplicated.
Each record can therefore be found in each of the copy statements, and the
total number of such duplications is in the hundreds for some tables.

What could be causing this? I've made only the following observations:
1. The nested loops always occur in the same spot within the outer copy
statements (if records have not changed between dumps) and are preceded by
the exact same column of the same record in each dump.
2. It is clear that these copy statements are NOT part of any record.
3. The same behaviour exists for insert style dumps (where there is no copy
command).
4. Each copy statement includes the entire table, NOT a selection of
records from it.

In years of using pg_dump I have never come across this issue.

--
Andy