"Anyone know why paying in slips require you to fill in the amount at least twice and write the name of the cheque signatory in a strip about 2 cm wide? "
IME, you only have to write the amount once for each cheque, and the total is written in the box on the front. If you're only paying in a single cheque, it might seem like duplicated effort, because then you really are writing the amount twice. (That's on the paying in slip itself, of course; if you're using a 'stub' format paying in book, rather than one in which carbon copies are kept) you have to write everything twice.
What I find annoying is that some banks' paying in books seem to unintentionally be designed for left-handed people. These are where the cheque details are written on the rear of the paying in slip, but it remains oriented the same as the front, so you're writing on the left hand side when writing on the rear. If you're paying in a lot of cheques (as you might if you are a business and your customers refuse to greet modern technology), you end up filling in both columns of cheque details - and the column closest to the spine, especially the amount, can be awkward to fill in with your right hand at certain depths in the paying in book.
FFS, print the reverse of the slips so that there is one (portrait) column, rather than two (landscape) columns.
"It's like they had 200 years supply of the things printed in 1950 and they have to use them all up."
That's my bank with cheque books on my business account.
My use of cheques is rare, and most years I write only a single cheque. Several years ago, I reached the point in a cheque book where a replacement is automatically sent. They sent two - each containing 100 cheques. And earlier this year they sent two more, for no apparent reason. I now have enough cheques to keep me going until something like 2410. Perhaps the late 2300s if I ever get extravagant and sometimes write two cheques in one year.