Doubles for Alternate Sounds Per Letter -- Preface to Actual Subject:
I am aware that, with the exception of the letters ess, thorn, eth (by extension, though not historically), b (older manuscripts*), and f (later manuscripts) [this list is all off the top of my head], if those appear in between vowels and voiced consonants, unless they be at the end of a syllable and then the start of another (e.g. "bookkeeper," in E.M.E. and Contemp. Eng.), then they make a voiceless sound [save for "b (older manuscripts)"].
*"b" was always /b/, but where "f" now makes the /v/ phoneme, "b" was used in some old manuscripts--such is to be expected.
Actual Subject of the Question:
I am aware that double consonants are supposed to be pronounced independently, except in the aforementioned case, but how is this done? Is this done the way we tend to do in spoken contemporary English, in sentences? E.g. "Ship pan."
There are two ways I can put across pronunciation of that: the way I hear most commonly here in America, and how I pronounce it, as something along the lines of "shim-pan." Not quite an /m/, not really, but an unreleased, slightly held /p/.
Or, as what I hear more commonly, it seems, though admit I don't hear it much, the received pronunciation, where one would hear along the lines of probably "shiph-pan;" "Ph" not standing for the aspirated consonant /pʰ/, but a normal /p/ with breath--the same breath would be replaced with a vowel, if one were to be.
So, for example, the word scieppan. Without using I.P.A., the way I would write I currently say it is "shim-pan," but "m" is not: /m/, as opposed to saying, again, as I would spell it: "shi-phpan."
In short, I cannot comprehend how two of the same plosive consonants can be next to each other without a vowel, or an extremely slight exhaling of air, like /h/, which, at least to me, should be written with an "h" or other equivalent letter.