I'm supposing that the imperfect here is inceptive "he started to write". Logically the second time in verse 8 would be "he continued to write", which doesn't seem to be inceptive. [In fact there seem to be a larger number of imperfects in this passage than there usually is in the same length of text.]
Is the use of the imperfect here significant?
John 8:6, 8 wrote:Τοῦτο δὲ ἔλεγον πειράζοντες αὐτόν , ἵνα ἔχωσιν κατηγορίαν κατ’ αὐτοῦ . Ὁ δὲ Ἰησοῦς κάτω κύψας, τῷ δακτύλῳ ἔγραφεν (NAUSB κατέγραφεν) εἰς τὴν γῆν.
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8 Καὶ πάλιν κάτω κύψας ἔγραφεν εἰς τὴν γῆν .