Hola:
¿Te sirve esto? ¿O necesitas cada síntoma en una columna?
> d <- data.frame(PatientID=c(1, 2, 3, 3, 3, 4, 5, 5, 2, 2, 3), Symptom=c('Obesity', 'Pancreatic_Neoplasms', 'Abdominal_Obesity', 'Pain', 'Chronic_Kidney_Failure', 'Ecoli_Infections', 'Sarcopenia', 'Inflammation', 'Obesity1', 'Obesity', 'Pancreatic_Neoplasms'), stringsAsFactors=FALSE)
> dd <- data.frame(PatientID=1:5, Symptom=unlist(lapply(1:5, FUN=function(i) paste(d[d$PatientID==i,2], collapse=','))))
> dd
PatientID Symptom
1 1 Obesity
2 2 Pancreatic_Neoplasms,Obesity1,Obesity
3 3 Abdominal_Obesity,Pain,Chronic_Kidney_Failure,Pancreatic_Neoplasms
4 4 Ecoli_Infections
5 5 Sarcopenia,Inflammation
Un saludo.