A convicted killer who stabbed another man to death in a Sydney shoes and accessories shop in broad daylight has been jailed for at least 20 years.

Douglas Johnson, 44, was found guilty by Justice Stephen Campbell in February of the murder of David Morrison, 41, with a short-bladed knife in a Campsie store in June 2017.

He was jailed on Friday for 26-and-a-half years with a non-parole period of 20 years, the judge noting Johnson had a 'baseless suspicion' his victim was stalking him and it was 'pure coincidence' they were in the same place that day.

Douglas Johnson (pictured) stabbed a man to death in a Sydney shoes and accessories shop

'He was completely unable to control his violent impulses involving the use of a knife to obtain redress in respect of, in this case, an imagined grievance,' Justice Campbell said in the New South Wales Supreme Court.

Johnson was also convicted of assaulting Mr Morrison's friend, Scott Campbell, immediately after the murder.

The judge said Mr Morrison was 'at least known to the offender by sight' and likely due to previous encounters.

Mr Johnson stabbed David Morrison with a knife in a store in Campsie (pictured)

Evidence before the judge-alone trial established Johnson's partner and Mr Campbell both attended the same methadone clinic in Campsie on the day of the murder.

'For no good reason whatsoever, the offender, and apparently his partner, jumped to the baseless conclusion Mr Morrison and Mr Campbell were stalking them with the intention of ambushing the offender,' the judge said.

He said Johnson strode into a shoe shop 'bent on confronting' Mr Morrison and uttered words to the effect of 'Are you conspiring against me?'.

Johnson then produced the knife and thrust it twice into Mr Morrison's left pelvic and upper thigh region, severing the 41-year-old's femoral artery.

He punched Mr Campbell before walking briskly out of the shop.

The judge noted Johnson had a 'baseless suspicion' his victim was stalking him and it was 'pure coincidence' they were in the same place that day.

Johnson was 18 when he fatally stabbed 25-year-old Peter John Miller in the upper back as he made his way home from a Newcastle hospital in June 1993.

Mr Miller had received stitches there for three non-fatal knife wounds earlier inflicted by Johnson during a fight that broke out between the pair, who were known to each other, while they were drinking heavily with a group.

Johnson made admissions in a police interview that he'd intended to kill Mr Miller, apparently motivated by some personal animosity.

He was convicted and jailed for the murder and released on parole under strict supervision in 2015.

On Friday, Johnson's second murder sentence was backdated, meaning he'll eligible for parole from September 2037.