Has anyone come across inaccuracies in block entries?
Yes - frequently.
If a family moved away for 7 years and then returned home, would they have the baptisms of children christened while away then entered en masse into their home parish register
It's quite possible, especially if some of them hadn't been recorded in their temporary parish of residence.
how accurate is the record likely to be
As accurate as their recollections, and the hearing of the parish clerk!
would one expect to find a "double entry" with another parish?
It's unusual, but not unheard of. Usually a double baptism record only occurs when the birth occurred outside the parents' parish of residence - for example a young woman sometimes returned to her mother's home for the birth of her first child, or the parents might be just living for a short time in a different parish.
I had always assumed that block entries occurred when the family hadn't baptised any of the children yet and then did them as a job lot
Just because they were
recorded as a job lot doesn't necessarily mean the children were all actually
baptised at the same time as a job lot! I've seen job lots with lists of entries along the lines of 'xxx and yyy had a child born on the such-and-such and baptised soon thereafter'.
in 1854, when the start of statutory civil registration was approaching, large numbers of baptism, some going back decades, were recorded retrospectively - in some parishes these records are in a separate section of the parish register called 'Register of Neglected Entries'.
