The classic pentecostal understanding is that upon reception of the Spirit baptism a believer is then indwelt by the Holy Ghost (and not before). Now, I personally have some misgivings about that particular aspect of traditional pentecostal belief (having come to believe the indwelling and [for lack of a better term] the infilling, to be two distinct operations of the Spirit). However, using that as a starting point ...
The idea of being filled with the Spirit, and it's relation to being baptized in the Spirit, seems to be initially synonymous. That is to say, one is initially filled with the Spirit when they are baptized in the Spirit. However, once baptized, the believer can (and should) experience re-fillings. The baptism of the Spirit is not said to be performed over and over, but our submission to Him and our being caught up in His presence, overflowing with His enabling, and exuding His power and gifting is something that reoccurs throughout our life in the Spirit.
So, after one is initially filled (i.e. Spirit baptized), they can be re-filled as oft as needed or necessary.
Simplistic, but adequate, no?

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