I have the same problem. In my source.list I had:
deb Index of /pool/PSA_18.0.34_5691 xenial extras
deb Index of /ubuntu/PHP56_17 xenial all
deb Index of /ubuntu/PHP70_17 xenial all
deb Index of /pool/PHP_7.1.33_96 xenial all
deb Index of /PHP73_17 xenial all
so I removed all except the...