Christening and Baptism are the same, and the Church of England doctrine is that it can be administered only once to any child.
The usual arrangement was for the baby and godparents to go to church. The godparents, speaking on behalf of the baby, renounced the devil, declared belief in the Christian creed and expressed the intention to obey God's will and commandments and walk in the same. Water was poured on the baby and the minister said " (baby's name) I baptise thee in the name of the Father and of the Son and of the Holy Ghost".
Sometimes, especially if the baby was weak, the water was poured and the "I babptise thee" etc said at home. The baby was then, as stated in the rubric in the Private Baptism service in the Book of Common Prayer
lawfully and sufficiently baptised and ought not to be baptised again. Yet nevertheless if the child do afterward live it is expedient that it be brought into the Church
and there are prayers for such an occasion and the godparents then speak on behalf of the child making the same declarations that would normally have been made at the baptism.
Records were not always well or consistently kept and it is possible that some records may show both the date of the private baptism and the date the child was subsequently received into the church (building). These would then appear as two separate entries in the register.
However it would not be usual for there to be any large gap.
There are however other reasons a child might be baptised more than once. As Ken Graham points out it is possible that a child might be christened in the Church of England but also by another denomination. It may be possible to see from the records you are examining whether this is the case, as they may be amalgamated. If so it is possible that the girls in a family were taken to their mother's denomination, in the place she grew up, as well as having been baptised in the parish church where they now lived. This would be irregular, and most other denominations would not knowingly re-do a baptism.
If the records you are examining are purely C of E, which is very likely the case, then social entitlement may be a factor. Many charities were limited to persons baptised within a particular parish. The Poor Law also required destitute persons to be maintained at the expense of the parish, usually where they were christened or had established some other connection. To be baptised in more than one parish was, of course, not something the Church would approve of, and the second one would be invalid if discovered. It is possible, for example, that there was some charitable provision for girls education in a particular parish.
Again however there would seem unlikely to be any gap of as long as 18 months since suspicions would be aroused as to why the child hadn't already been baptised.
I suspect however that the most likely cause of the duplicate baptisms in the records is that the records may include both originals and transcripts.
Every christening, marriage and burial was entered in the Parish Records. Every year the churchwardens were responsible for making transcripts. A transcript was a copy of every entry in the past year, and they had to be sent to the bishop, or to the archdeacon, or both, and in some cases to other authorities. The fact cited by OP that his data usually has the second event 6 to 18 months after the first is suggestive. The practice in the locality may have been to send a full years transcript within six months of the end of the year, or at least that is when the Archdeacon etc entered them in his own records.
It is possible that your data includes parish records and transcripts, with only the date of notification in the transcript. Not all parish records have survived and so sometimes the transcript may be the only record.
Where the same date is given for a christening/baptism service in more than one church, it is likely that the service occured in a chapel of ease or district chapel. These are official Church of England churches, but which are not the main church of the parish. In such cases the event was recorded immediately in the log book of the church where it occurred and then as soon as convenient afterwards would also be entered in the parish records, maintained at the parish church. In this way the event would be recorded in two plavces (quite apart from subsequent transcription). But there wouldonly have been one event. This was particularly common in the north where parishes tended to be larger.
