He was also the target of alleged murder-for-hire conspiracy concocted by Leonard from inside the Marion County Jail.
Monday, the two former friends faced off in a basement courtroom in the St. Joseph County Courthouse.
"Were you and Mark Leonard friends?" Deputy Prosecutor Denise Robinson asked the witness.
"I thought we were," said Duckworth.
"'Leonard don't have anything to lose now to kill me because I know too much,'" Duckworth recalled telling his father, a retired Indianapolis Police Department (IPD) officer.
Before Duckworth took the stand, the Defense reminded Judge John Marnocha that he excluded testimony that would detail Leonard's penchant for scamming women during casino visits, convincing them to write him checks to cover costs of his floundering Mastercraft Restoration business, allegations confirmed by FOX59 News.
Duckworth would also be blocked from talking about another insurance scam involving a motorcycle as well as Leonard's ejection from a Shelbyville casino and the suspect's mental and emotional dominance over his then-girlfriend Monserrate Shirley.
During the opening day of the final week of testimony in the Richmond Hill trial, a veteran homicide detective told jurors how he began piecing together the conspiracy to cover up the fatal November 10, 2012 explosion.
Mark Leonard faces more than 50 counts for the murderous insurance fraud conspiracy that destroyed then-girlfriend Monserrate Shirley's home, killed two neighbors and led to an alleged "hit man" plot to kill Duckworth.
Duckworth told jurors that Leonard told him a few days before the explosion that he was shopping on-line for a Ferrari.
"'How come? How can you afford a Ferrari?'" Duckworth said he asked Leonard. "'Where did you get the money to buy a Ferrari?'"
"'The tsunami winds blew out the fireplace and the house blew up and they were going to get $300,000,'" Duckworth recalled Leonard answering.
"What did you say?" asked Robinson.
"There wasn't much else to say," said Duckworth. "It was like, 'Alright.'"
Duckworth said Leonard later told him he made up the story about the tsunami winds.
Prosecutors then displayed a sketch they said Leonard made inside the Marion County Jail as part of the "hit man" plot.
Duckworth identified the drawing as a sketch of his house and the streets surrounding it.
Justin Leonard followed Duckworth to the stand, recalling that he had been instructed by his father, Bob Leonard, Jr., to retrieve several items, including totes, photographs, figurines and golf clubs that investigators claim came from Shirley's house.
"As the jury's aware, the information about the totes and golf clubs, those were items of property that came out of the Shirley residence and those ended up in Bob Leonard's trailer," said Robinson.
Indianapolis Metropolitan Police Department (IMPD) Homicide Detective Jeff Wager identified several records detailing calls and text messages sent to and from phones belonging to Leonard, Shirley and Duckworth.
Within a day of the explosion, Duckworth sent a message to Leonard that read, "I'm pretty sure you made the news last night."
Wager testified that he interviewed three people, including Mark Leonard's nephew Justin Leonard, and each witness corroborated the other's observations about items removed from Shirley's house and stashed at the trailer of Bob Leonard, Jr., on the southwest side of Indianapolis.
The Prosecution then introduced photographs of three storage totes that were said to be filled with Shirley's personal effects removed from the house before the explosion and a set of golf clubs belonging to John Shirley which he left at his ex-wife's home when he moved out.
With the testimony phase of the trial coming to an end, both sides are now preparing for their closing arguments and jury instructions.
Judge Marnocha indicated he has received the State's proposed jury instructions, which will undoubtedly include a tutorial on the Felony Murder charge which allows for conviction on whether a suspect should have known his actions would lead to death, even if that was not his intention.
The Defense told the Court it was still preparing its proposed jury instructions, including two versions of the reckless homicide statute which could lead jurors to find their client guilty of charges less than felony murder.
"We have a little bit of evidence yet to present. Some state farm evidence that the jury will hear about the insurance policies, about claims that were made, about payments that were made, that sort of thing, some DNA evidence they will hear about tomorrow, the DNA evidence from the front door of the Shirley residence," said Robinson.
Judge Marnocha indicated that he was also considering instructions to the jurors on lesser included charges which could give the panel the option of finding Leonard guilty of causing the deaths of Jennifer and Dion Longworth but not murder.
"Well whether I've proven it or not will be up for the jury to decide, but we think that all the pieces to the puzzle have been or will be presented to the jury," said Robinson when asked if the State has proven their cast. "By the time we rest for them to draw that conclusion, yes."
If convicted of murder, Leonard faces the possibility of spending life in prison without parole.