A Phase 2 Study Comparing 2 Intermittent Dosing Schedules of Duvelisib in Participants With Indolent Non-Hodgkin Lymphoma

Inclusion Criteria: * Eastern Cooperative Oncology Group performance status ≤ 2 * Histologically confirmed diagnosis of iNHL (subtypes include follicular lymphoma \[FL\] Grades 1 to 3a), marginal zone lymphoma (splenic, nodal, or extranodal), or small lymphocytic lymphoma * Must have received 1 prior systemic regimen for iNHL * Must have documented radiologic evidence of disease progression, at least 1 bi-dimensionally measurable lesion ≥ 1.5 centimeters (which has not been previously irradiated), according to 2007 revised International Working Group criteria, and be a candidate for a subsequent line of therapy. * Must have adequate organ function defined by the following laboratory parameters: * Absolute neutrophil count ≥ 1.0 × 10\^9/liter (L) * Platelet count ≥ 75 × 10\^9/L * Hemoglobin ≥ 8 grams/deciliter * Estimated creatinine clearance ≥ 60 milliliters/minute, as determined by the Cockcroft-Gault method * Total bilirubin ≤ 1.5 × upper limit of normal (ULN) (exception: participants with Gilbert's Syndrome may have a bilirubin \> 1.5 × ULN) * Aspartate transaminase/serum glutamic-oxaloacetic transaminase and alanine aminotransferase/serum pyruvic transaminase ≤ 3.0 × ULN Exclusion Criteria: * Anticancer treatment, major surgery, or use of any investigational drug within 28 days before the start of study intervention; palliative radiation therapy is allowed if \> 7 days before planned first dose of study interventions, and any toxicity is Grade ≤ 1 * Clinical or histological evidence of transformation to a more aggressive subtype of lymphoma or grade 3b FL or Richters' transformation or chronic lymphocytic leukemia * Prior allogeneic hematopoietic stem cell transplant; prior treatment with a PI3K inhibitor * History of drug-induced colitis or pneumonitis; tuberculosis treatment ≤ 2 years prior to randomization; administration of a live or live attenuated vaccine within 6 weeks of randomization * Ongoing treatment with chronic immunosuppressants or systemic steroids or treatment for systemic bacterial, fungal, or viral infection * Active cytomegalovirus or Epstein-Barr virus infection * Unable to receive prophylactic treatment for pneumocystis, herpes simplex virus, or herpes zoster at screening * Concurrent administration of medications or foods that are strong inhibitors or inducers of cytochrome P450 3A. No prior use within 2 weeks before the start of study intervention. * Baseline QT interval corrected with Fridericia's method \> 500 milliseconds * Concurrent active malignancy other than non-melanoma skin cancer or carcinoma in situ of the cervix, bladder cancer, or prostate cancer not requiring treatment. Participants with previous malignancies are eligible if they have been disease-free for 2 years or more. * Unstable or severe uncontrolled medical condition that would, in the Investigator's judgment, increase the participant's risk to participating in this study.